Pastes for solar cells, solar cells, and methods of making same
12243950 ยท 2025-03-04
Assignee
Inventors
Cpc classification
H10F77/223
ELECTRICITY
International classification
Abstract
A paste (32) for use in metallization of a solar cell (12) includes an organic vehicle (44) and a mixture of copper-containing particles (46), metal-oxide-containing nanoparticles (50), and secondary oxide particles (52) different from the metal-oxide-containing nanoparticles (50). The secondary oxide particles (52) include particles (42) of a metal oxide and a metal of the metal oxide capable of reducing at least some of the metal-oxide-containing nanoparticles (50) to metal when heated. The organic vehicle (44) is capable of reducing the metal oxide of the secondary oxide particles (52) upon decomposition of the organic vehicle (44). A paste (32) includes a mixture of particles (42) including metallic copper particles (46), nanoparticles (50), and metal oxide particles (52) in the organic vehicle (44). The nanoparticles (50) include at least one oxide of nickel, copper, cobalt, manganese, and lead. The metal oxide of the metal oxide particles (52) has a more negative Gibbs Free Energy of Formation than a metal oxide of the at least one oxide of the nanoparticles (50).
Claims
1. A paste for use in metallization of a solar cell, the paste comprising: an organic vehicle including polyvinylpyrrolidone, and a mixture of copper-containing particles, metal-oxide-containing nanoparticles, and secondary oxide particles in the organic vehicle, wherein the metal-oxide-containing nanoparticles include nanoparticles consisting of a mixed oxide selected from oxides of (i) nickel and lead; (ii) manganese and lead; (iii) copper, nickel, and lead; (iv) copper, silver, nickel, and lead; (v) cobalt and lead; (vi) nickel, cobalt, and lead; or (vii) nickel, silver, and lead, or (viii) a mixture of two or more nanoparticles (i)-(vii), and wherein the secondary oxide particles include particles of a metal oxide, and a metal of the metal oxide is capable of reducing at least some of the metal-oxide-containing nanoparticles to metal when the paste is heated to a temperature at which the organic vehicle decomposes.
2. The paste of claim 1 wherein the copper-containing particles include metallic copper particles, copper-containing alloy particles, or copper-containing non-metallic particles or a mixture of two or more thereof.
3. The paste of claim 2 wherein the copper-containing particles have a plurality of metallic particles and/or a plurality of oxide/hydroxide particles on a surface thereof.
4. The paste of claim 3 wherein the plurality of oxide/hydroxide particles are metal oxide particles in which the metal of the metal oxide is selected from nickel, cobalt, and manganese or combinations thereof.
5. The paste of claim 3 wherein the plurality of metallic particles and/or the plurality of oxide/hydroxide particles on the surface of the copper-containing particles are doped with at least phosphorous.
6. The paste of claim 3 wherein the surfaces of the copper-containing particles are not fully covered by the plurality of metallic particles and/or the plurality of oxide/hydroxide particles.
7. The paste of claim 1 wherein the secondary oxide particles include particles consisting of antimony oxide.
8. The paste of claim 1 wherein the metal-oxide-containing nanoparticles consist of a mixed oxide of nickel and lead.
9. The paste of claim 1 wherein a ratio of secondary oxide particles to metal-oxide-containing nanoparticles is in a range of 0.50 to 16.96 by weight.
10. The paste of claim 3 wherein the plurality of metallic particles include one metal selected from nickel, cobalt, and manganese or a combination thereof and the copper-containing particles with the plurality of metallic particles have a surface area in the range of 0.22 m.sup.2/g to 5 m.sup.2/g.
11. A paste comprising: an organic vehicle including polyvinylpyrrolidone, and a mixture of particles including metallic copper particles, nanoparticles, and metal oxide particles in the organic vehicle, wherein the nanoparticles include nanoparticles consisting of a mixed oxide selected from oxides of (i) nickel and lead; (ii) manganese and lead; (iii) copper, nickel, and lead; (iv) copper, silver, nickel, and lead; (v) cobalt and lead; (vi) nickel, cobalt, and lead; or (vii) nickel, silver, and lead, or (viii) a mixture of two or more nanoparticles (i)-(vii), and wherein the metal oxide particles include a metal oxide selected from oxides of antimony, tin, tellurium, manganese, phosphorous, bismuth, chromium, aluminum, cobalt, copper, and nickel or combinations thereof.
12. The paste of claim 11 wherein the metal oxide of the metal oxide particles has a more negative Gibbs Free Energy of Formation than the mixed metal oxide of the nanoparticles.
13. A solar cell made with the paste of claim 1.
14. The paste of claim 1 wherein the metal oxide of the secondary oxide particles is one or more of antimony oxide, tin oxide, tellurium oxide, manganese oxide, phosphorous oxide, bismuth oxide, chromium oxide, and aluminum oxide.
15. The paste of claim 1 wherein all dimensions of the metal-oxide-containing nanoparticles are equal to or less than 100 nm.
16. The paste of claim 11 wherein all dimensions of the nanoparticles are equal to or less than 100 nm.
17. The paste of claim 11 wherein the metal oxide of the metal oxide particles is selected from oxides of antimony, tin, tellurium, manganese, phosphorous, bismuth, chromium, and aluminum or combinations thereof.
18. The paste of claim 11 wherein the metallic copper particles have a plurality of metallic particles and/or a plurality of oxide/hydroxide particles on a surface thereof.
19. The paste of claim 18 wherein the plurality of oxide/hydroxide particles are metal oxide particles in which the metal of the metal oxide is selected from nickel, cobalt, and manganese or combinations thereof.
20. The paste of claim 18 wherein the plurality of metallic particles and/or the plurality of oxide/hydroxide particles on the surface of the metallic copper particles are doped with at least phosphorous.
21. The paste of claim 18 wherein the surfaces of the metallic copper particles are not fully covered by the plurality of metallic particles and/or the plurality of oxide/hydroxide particles.
22. The paste of claim 18 wherein the plurality of metallic particles and/or a plurality of oxide/hydroxide particles include one metal selected from nickel, cobalt, and manganese or a combination thereof and the metallic copper particles with the plurality of metallic particles and/or a plurality of oxide/hydroxide particles have a surface area in the range of 0.22 m.sup.2/g to 5 m.sup.2/g.
23. The paste of claim 11 wherein a ratio of metal oxide particles to nanoparticles is in a range of 0.50 to 16.96 by weight.
24. The paste of claim 11 wherein the metal oxide particles include particles consisting of antimony oxide.
25. A solar cell made with the paste of claim 11.
26. A paste for use in metallization of a solar cell, the paste comprising: an organic vehicle, and a mixture of copper-containing particles, metal-oxide-containing nanoparticles, and secondary oxide particles in the organic vehicle, wherein the metal-oxide-containing nanoparticles include nanoparticles consisting of a mixed oxide selected from oxides of (i) nickel and lead; (ii) manganese and lead; (iii) copper, nickel, and lead; (iv) copper, silver, nickel, and lead; (v) cobalt and lead; (vi) nickel, cobalt, and lead; or (vii) nickel, silver, and lead, or (viii) a mixture of two or more nanoparticles (i)-(vii), and wherein the secondary oxide particles include particles consisting of antimony oxide and antimony is capable of reducing at least some of the metal-oxide-containing nanoparticles to metal when the paste is heated to a temperature at which the organic vehicle decomposes.
27. A paste comprising: an organic vehicle, and a mixture of particles including metallic copper particles, nanoparticles, and metal oxide particles in the organic vehicle, wherein the nanoparticles include nanoparticles consisting of a mixed oxide selected from oxides of (i) nickel and lead; (ii) manganese and lead; (iii) copper, nickel, and lead; (iv) copper, silver, nickel, and lead; (v) cobalt and lead; (vi) nickel, cobalt, and lead; or (vii) nickel, silver, and lead, or (viii) a mixture of two or more nanoparticles (i)-(vii), and wherein the metal oxide particles include particles consisting of antimony oxide.
28. The paste of claim 1 wherein the copper-containing particles consist essentially of copper.
29. The paste of claim 26 wherein the copper-containing particles consist essentially of copper.
30. The paste of claim 1 wherein the paste is from 62 wt. % to 98 wt. % copper metal.
31. The paste of claim 11 wherein the paste is from 62 wt. % to 98 wt. % copper metal.
32. The paste of claim 26 wherein the paste is from 62 wt. % to 98 wt. % copper metal.
33. The paste of claim 27 wherein the paste is from 62 wt. % to 98 wt. % copper metal.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the detailed description given below, serve to explain various aspects of the invention.
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DETAILED DESCRIPTION
(54) With reference to
(55) As is described herein, one or more of the cells 12 may include one or more electrical conductors (i.e., electrodes) containing copper. The largest proportion of the metal conductors may be copper with the remaining proportion being of other metals and/or metal-containing materials. In one embodiment, one or more of the solar cells 12 is free of silver. Advantageously, the solar cells 12 may be produced in a cost-effective manner as copper-containing conductors replace silver conductors. Furthermore, as is described herein, the copper-containing conductors are produced from a copper-containing paste in a metallization process. As a further advantage, the pastes of the present invention may be substituted into processes that currently utilize silver-containing pastes, such as solar cell/panel manufacturing processes. As will be appreciated, this eliminates the necessity of large capital investments observed with copper electroplating techniques while providing a cost-effective process for producing a solar cell having predominately copper electrodes and in which silver is absent.
(56) With reference to
(57) A plurality of electrical conductors 26, referred to as fingers, are exposed at the front surface 22 of the solar cell 12. As shown in
(58) In that regard and with reference now to
(59) During printing on the substrate 40, and with reference to
(60) By way of example only, and not limitation, the organic vehicle 44 may be a mixture of organic binders together with one or more solvents, dispersants, viscosity modifiers and/or surface tension modifiers. Exemplary binders include polyvinylpyrrolidone (PVP), ethyl cellulose, and monosaccharides, such as fructose. Exemplary solvents include alcohols, ester alcohols, and glycols, such as propylene glycol, ethylene glycol, polyethylene glycol, Texanol ester alcohol, as well as ether alcohols and glycols, such as diethylene glycol and triethylene glycols, in addition to water. Exemplary viscosity and surface tension modifiers include DISPERBYK 111, DISPERBYK 180, DISPERBYK 190, RHODAFAC RE-610, and sodium stearate, to name only a few. The volume of the organic vehicle 44 in the paste 32 and the proportion of each type of particle 42 can be tuned to meet a desired rheological (e.g. viscosity) specification. In general, the paste 32 may comprise of only a few weight percent organic vehicle 44. By way of example, the paste 32 includes the organic vehicle 44 in a range from 1 wt. % to 15 wt. %.
(61) The copper-containing paste 32 is a mixture of particles of differing composition. That is, the paste 32 may be a mixture of powders that provide all the particles 42. Each one of the powders may include particles 42 having different composition, particle size, and/or particle shape.
(62) In one embodiment, the copper-containing paste 32 includes a plurality of copper-containing particles 46 that form the largest portion by volume of the particles 42 in the paste 32. The copper-containing particles 46 may form the majority volume of the fingers 26. With the make-up of the particles 42, following firing, the fingers 26 are capable of providing electrical conductivity, via metallic copper or copper-containing metal, sufficient to promote efficient solar cell performance. By way of example, the copper-containing particles 46 may be metallic copper and form from 48 vol. % to 88 vol. % of the solids in the paste 32. This may translate into at least 80 wt. % copper metal in the paste 32 before drying. The range of copper metal in the paste 32 may be from 62 wt. % to 98 wt. %. While the copper-containing particles 46 are described with reference to metallic copper, the copper-containing particles 46 are not limited to metallic copper, which may include minor impurities elements. The copper-containing particles 46 may be particles of a copper-containing alloy or copper-containing non-metallic (e.g., oxide) particles capable of being reduced to copper or to a copper-containing alloy. Copper-containing alloys are those that contain copper as their principal component, for example, bronze (a copper-tin alloy) and brass (a copper-zinc alloy).
(63) As further examples, copper-alloys may include copper in combination with other metal elements including nickel, aluminum, and/or lead. Copper-phosphorous alloys are also contemplated. The copper-containing particles 46 may be a mixture of two or more of metallic copper, copper-containing alloy, and non-metallic copper-containing material.
(64) The copper-containing particles 46 are provided in the paste 32 by a powder that is described by a particle size distribution. By way of example, a particle size distribution for one copper powder may include particles in a range of from 10 m to 22 m in size. That is, the powder contains particles from 10 m in size up to 22 m in size with an average particle size in the range. By way of further example, a narrower particle size distribution of copper-containing particles 46 includes particles ranging from 10 m to 20 m in size. Other exemplary examples include progressively narrower particle size distributions, particle size distributions with particles in the range of 7.5 m to 14.7 m (these are D.sub.10 and D.sub.90, respectively), 1.8 m to 7.7 m (these are D.sub.10 and D.sub.90, respectively), 1.2 m to 3.2 m (these are D.sub.10 and D.sub.90, respectively), or 0.2 m to 1.6 m (these are D.sub.10 and D.sub.90, respectively). The copper-containing particles 46 may be particles from one powder having a particle size distribution (e.g., one of the exemplary distributions above), or a mixture of multiple different powders such that there are multiple distinct particle size distributions mixed in the paste 32. The particle size distribution correlates with the surface area per unit weight of the powder. Thus, as the particle size distribution is shifted to smaller particles, the surface area per gram of the powder increases.
(65) For example, a powder of coarse copper-containing particles 46 may be mixed with a powder of fine copper-containing particles 46. The fine copper-containing powder 46 may fill in the interstices between the larger copper-containing particles 46 in the paste 32. This results in a bimodal particle size distribution of copper-containing particles 46 in the paste 32 and may maximize the volume of copper in the paste 32. In this way, combinations of any two or more of the particle size distributions identified above may be mixed together to adjust the volume percentage of copper in the paste 32 to be within a desired range. For a binary mixture, the ratio of different sized copper-containing powders by weight may range from 60 wt. % coarse-40 wt. % fine to 90 wt. % coarse-10 wt. % fine. For example, a coarse copper powder having a particle size distribution with a range in particle size from 10 m to 20 m may be mixed with a fine copper powder having a particle size distribution with a range of 1.2 m to 3.2 m at a ratio of 90 wt. % coarse to 10 wt. % fine. Other ratios of two or more copper powders are possible, for example, a coarse copper powder having a particle size distribution with a range of particles sizes from 7.5 m to 14.7 m may be mixed with a fine copper powder having a particle size distribution with a range of particle sizes from 1.2 m to 3.2 m at a ratio of 70 wt. % coarse to 30 wt. % fine. As an example of four different copper-containing powders, a particle size distribution with a range of particle sizes from 10 m to 22 m is mixed with three other copper-containing powders having particle size distributions with ranges from 7.5 m to 14.7 m, from 1.8 m to 7.7 m, and from 1.2 m to 3.2 m at a weight ratio of 37 to 21 to 12 to 30, respectively. Copper-containing powders according to embodiments of the invention are commercially available from the Bean Town (copper flakes), Chemical Store (item Cu112SP), ACuInternational (items 610 and 603), and Ames Goldsmith (item C1-2000).
(66) It will be appreciated that the copper-containing paste 32 in
(67) Referring to
(68) The nanoparticles 50 are included in the paste 32 in an amount sufficient to reduce or eliminate oxidation of metallic copper or copper alloy either in the paste 32 or formed during manufacturing of the solar cell 12, described further below, while also reducing copper diffusion into the wafer 14 during use of the solar cell 12. The nanoparticles 50 may also enable etching of the anti-reflective coating 20 and permit an electrical connection of the copper with the wafer 14. Reduction in copper diffusion may be by way of forming a barrier between copper of the fingers 26 and the wafer 14 during manufacturing. Without being bound by any theory, the nanoparticles 50 are believed to participate in the formation of the barrier so that elements of the nanoparticles 50 form an interface between the silicon of the wafer 14 and the copper from the copper-containing particles 46. By way of example, the nanoparticles 50 may be present in an amount up to 17 wt. % in the paste 32. And, as further examples, the nanoparticles 50 may be present in a detectable amount up to 5 wt. %; a detectable amount up to 3 wt. %; from about 0.2 wt. % (or near a lower limit of detection) up to 3 wt. %; or from about 0.2 wt. % up to 5 wt. % in the paste 32. Mixtures of different nanoparticles are also contemplated with the total weight of the mixture falling within the ranges above. For example, a mixture of nanoparticles 50 may be 50 wt. % Ni/Ni/Pb nanoparticles and 50 wt. % Cu/Ni/Pb nanoparticles.
(69) As shown in
(70) The copper-containing particles 46 may be coated with particles 56 according to a process shown in
(71) With continued reference to
(72) With regard to reducing oxides in the paste 32, the secondary oxide particles 52 may be one or more metal oxides having a more negative Gibbs Free Energy of Formation than at least one of the oxides of the nanoparticles 50 and oxides of the particles 56. By way of example, the secondary oxide particles 52 may have a greater negative Gibbs Free Energy of Formation than at least one of copper oxide (Cu.sub.2O, CuO), lead oxide (PbO, PbO.sub.2), or nickel oxide (NiO) at the temperatures to which the wafer 14 and paste 32 are heated during manufacturing. In one embodiment, the secondary oxide particles 52 are antimony oxide (Sb.sub.2O.sub.3) particles. Other secondary oxide particles 52 include, but are not limited to tin oxide (SnO, SnO.sub.2), tellurium oxide (TeO), manganese oxide (MnO, MnO.sub.2), phosphorous oxides (P.sub.4O.sub.10, PO.sub.2), bismuth oxide (Bi.sub.2O.sub.3), chromium oxide (Cr.sub.2O.sub.3), and aluminum oxide (Al.sub.2O.sub.3). The use of these oxides to reduce other oxides in the paste 32 may depend on the oxidation state of the secondary oxide particles 52, the melting point temperature of the secondary oxide particles 52, the processing temperature and the partial pressure of a reducing environment surrounding the secondary oxide particles 52. Further, the secondary oxide particles 52 may include frit particles, such as lead-containing silicate glasses or other metal-containing glasses that contain one or more oxide components that have a more negative Gibbs Free Energy of Formation than at least one of Cu.sub.2O, CuO, PbO.sub.2, PbO, and NiO. Exemplary frit particles include borate glasses, such as B.sub.2O.sub.3PbOZnO glass, and silicate glasses, such as PbOSiO.sub.2B.sub.2O.sub.3Al.sub.2O.sub.3 glass.
(73) The paste 32 may include from 1 wt. % to 15 wt. % of the secondary oxide particles 52. The secondary oxide particles 52 may have a surface area in a range of 2 m.sup.2/g to 2.5 m.sup.2/g. A ratio of secondary oxide particles 52 to the nanoparticles 50 may determine the amount of oxide reduced and/or the amount of sintering aid that develops during heating. In one embodiment, a ratio of secondary oxide particles 52 to nanoparticles 50 is in a range of 0.50 to 16.96 by weight. In another embodiment, a ratio of secondary oxide particles 52 to metal-oxide-containing nanoparticles is in a range of 1.50 to 3.51 by weight. As used herein, consisting essentially of means that no other elements or particles are intentionally added to the paste. However, impurity content of other elements in oxide or metallic form from powders or the fabrication process are contemplated.
(74) With reference to
(75) Following drying, the particles 42 of the paste 32 are fired in a metallization process to produce the fingers 26 and/or the backside conductor 30. In one embodiment, manufacturing of the solar cell 12, including drying and firing, is completed in air. That is, the atmosphere during drying and firing is not controlled or modified by addition of inert or reducing gases. During firing, the particles 42 are exposed to temperatures from 600 C. to 930 C. in air for from 0.8 s to 10 s depending on the temperature profile of the furnace, belt speed and the composition of the paste 32. At these temperatures and times, in one embodiment, at least one of the organic binders remaining from the organic vehicle 44 decomposes and forms a reducing environment localized around the particles 42. It is believed that this environment reduces the secondary oxide particles 52. Reduction of at least some of the secondary oxide particles 52 produces at least one metal from the secondary oxide particle 52. For example, the composition of the organic vehicle 44 is believed to reduce antimony oxide (Sb.sub.2O.sub.3) to metallic antimony (Sb). As an additional example, the organic vehicle 44 may also reduce lead oxide to lead (Pb).
(76) In turn, the metal from the secondary oxide particle 52 reduces at least some of the nanoparticles 50 and/or the particles 56 on the copper-containing particles 46. Continuing the example with Sb, the metallic Sb generated reduces oxides of the nanoparticles 50 and of the particles 56. Where PbO is present, the metallic Sb may generate Pb by PbO reduction, and Pb may assist in the reduction of copper oxide. Thus, by the characteristics of particles 42 in the paste 32, a cascade-type reduction reaction may be precipitated by decomposition of the organic vehicle 44. In that cascade of reduction, one oxide is reduced to metal. That metal in turn reduces another oxide in the mixture of particles 42. Depending on the temperature; the composition and the volume of nanoparticles 50, secondary oxide particles 52, and particles 56; and the proportion of the organic vehicle 44 in the paste 32, the cascade reduction reaction may reduce all oxides in the paste 32 to their constituent metals. This may include any copper oxide present in the paste 32. In that case, a paste containing a large portion of metal oxides, contains a composite of different metals.
(77) At the temperatures at which the particles 42 are exposed during firing, reducing the nanoparticles 50 and the secondary oxide particles 52 may form a liquid sintering aid and may reduce and/or coat the copper-containing particles 46. The metal from the nanoparticles 50 and the secondary oxide particles 52 may react with the anti-reflective coating 20. This combination of particles is thought to produce an etchant.
(78) As shown in
(79) While the paste 32 initially includes oxide particles 50 and 52, the finger 26 is metallic as some or all the oxide particles are reduced during firing of the paste 32. The finger 26 may be free of internal oxides, excepting that the outer, air exposed surface of the finger 26 may form a native oxide (e.g., CuO and/or Cu.sub.2O, Sb.sub.2O.sub.3, etc.) when exposed to air. During sintering, particles and reaction products, in particular the metal from the nanoparticles 50 and the metal from the secondary oxide particles 52 together with the copper-containing particles 46, penetrate the anti-reflective coating 20. During sintering, the metals, nanoparticles 50, and/or secondary oxide particles 52 and/or the metals of those oxide particles contact the anti-reflective coating 20 and may produce reaction products of the metal and the material of the coating 20. For example, where the anti-reflective coating 20 is SiN, reaction products may include metal silicides and/or metal nitrides. These reaction products may form an interface 58 (generally indicated in
(80) As described with reference to
(81) In the exemplary process illustrated in
(82) Where additional coatings are desired over a copper oxide/hydroxide core, additional streams are added to the reactor 60 containing the copper oxide/hydroxide nanoparticles. For example, where a nickel oxide coating is desired over the copper oxide/hydroxide nanoparticles, a third stream 66 of aqueous nickel chloride and aqueous sodium borohydride is added to the reactor 60 and stirred. This coats the copper oxide/hydroxide nanoparticles with a layer of nickel oxide. A third layer of lead oxide/hydroxide is coated over the layer of nickel oxide by addition of a fourth stream 68 of aqueous lead nitrate to the reactor 60. This produces a lead oxide/hydroxide coating over the nickel oxide.
(83) The following are the possible reactions and side reactions occurring during the synthesis illustrated in
(84) Primary stoichiometric reaction occurring in a Reactor.
2Cu(NO.sub.3).sub.2(aq)+4NaBH.sub.4(aq).fwdarw.2Cu(s)+4Na(NO.sub.3)(aq)+2B(OH).sub.3(aq)+H2(g)+B.sub.2H.sub.6(g)Equation 1.
(85) Assumes excess NaBH.sub.4 reacts with H.sub.2O. In one process, lower than stoichiometric amounts are utilized.
NaBH.sub.4(aq)+4H.sub.2O.fwdarw.NaB(OH).sub.4(aq)+4H.sub.2(g)Equation 2.
(86) B.sub.2H.sub.6 is assumed to completely react with H.sub.2O and all the Cu reduces to Cu.sub.2O with the addition of NiCl.sub.2.
B.sub.2H.sub.6(g)+6H.sub.2O(l).fwdarw.2B(OH).sub.3(aq)+6H.sub.2(g)Equation 3.
Equation 4.
(87) Cu has a multi-valence nature, so can react with Cu.sup.2+ ions.
Cu(s)+Cu.sup.2+(aq).fwdarw.2Cu.sup.+(aq)Equation 4a.
(88) Cu.sup.+ can react with Cl.sup. ions.
Cu.sup.+(aq)+(aq)CuCl(s)Equation 4b.
(89) The CuCl can form Cu.sub.2O.
2CuCl(s)+H.sub.2O(l).fwdarw.2HCl(aq)+Cu.sub.2O(s)Equation 4c.
(90) The NiCl.sub.2 reacts with NaBH.sub.4 to coat the Cu.sub.2O particles with Ni.
2NiCl.sub.2(aq)+4NaBH.sub.4(aq).fwdarw.2Ni(s)+4NaCl(aq)+2B(OH).sub.3(aq)+H.sub.2(g)+B.sub.2H.sub.6(g)Equation 5.
(91) A substitution reaction occurs when Pb(NO.sub.3).sub.3 is added.
3Ni(s)+2Pb(NO.sub.3).sub.3(aq).fwdarw.3Ni(NO.sub.3).sub.2(aq)+2Pb(s)Equation 6.
(92) Both the Ni and Pb shells may oxidize to NiO and PbO. Less than stoichiometric amounts may be used in the reaction. If so, NaBH.sub.4 is assumed to become the limiting factor, and Equation 1 becomes Equation 7, and Equation 5 becomes Equation 8.
(93) Non-stoichiometric reaction occurring in Batch Reactor.
0.37Cu(NO.sub.3).sub.2(aq)+0.74NaBH.sub.4(aq).fwdarw.0.37Cu(s)+0.74Na(NO.sub.3)(aq)+0.37B(OH).sub.3(aq)+0.185H.sub.2(g)+0.185B.sub.2H.sub.6(g)Equation 7.
(94) The mass balance also assumes no excess NaBH.sub.4 reacts with H.sub.2O. The reaction in Equation 2 is assumed not to occur.
(95) Non-stoichiometric reaction for NiCl.sub.2 reacting with NaBH.sub.4 to coat the Cu.sub.2O particles with Ni.
0.1NiCl.sub.2(aq)+0.2NaBH.sub.4(aq).fwdarw.0.1Ni(s)+0.2NaCl(aq)+0.1B(OH).sub.3(aq)+0.05H.sub.2(g)+0.05B.sub.2H.sub.6(g)Equation 8.
(96) Substitution reaction occurs when Pb(NO.sub.3).sub.2 is added.
0.1Ni(s)+0.2Pb(NO.sub.3).sub.3(aq).fwdarw.0.1Ni(NO.sub.3).sub.2(aq)+0.07Pb(s)+0.13Pb(NO.sub.3).sub.3(aq)Equation 9.
(97) For non-stoichiometric amounts, Equation 3 becomes Equation 10.
(98) Reaction of B.sub.2H.sub.6 with H.sub.2O.
0.235B.sub.2H.sub.6(g)+1.41H.sub.2O(l).fwdarw.0.47B(OH).sub.3(aq)+1.41H.sub.2(g)Equation 10.
(99) Following the additions and reactions, at 70, the nanoparticles 50 are separated from the supernatant by centrifugation. And, at 72, the solid residue, which are the Cu/Ni/Pb nanoparticles, is dried. The nanoparticles 50 may be crushed to break apart agglomerates and then sieved and are ready for use in the paste 32.
(100)
(101) A similar process is illustrated in
(102) With reference to
(103) While a nickel salt is described with reference to
(104) Each of the particles 42 described above is then mixed with the organic vehicle 44. With reference to
(105) Once all the vehicle 44 and particles 42 have been added to the vacuum mixer 92, the mixture is mixed for a prescribed time under vacuum. At 102, the mixture is then milled. This may include using a three-roll mill, ball mill or other particle dispersing equipment. The milling process helps to break up particle agglomerates and facilitates homogenization and deaeration of the mixture. Following roll milling, at 104, the mixture is aged at elevated temperatures (e.g., 100 C.) for a prescribed length of time to improve wetting of the organic vehicle 44 on the particles 42. The mixture may be subsequently subjected to another round of roll milling at 106 and additional organic vehicle 44 (not shown) may be added to achieve a predetermined viscosity before the paste 32 is ready for use.
(106) In order to facilitate a more complete understanding of the invention, the following non-limiting examples are provided.
EXAMPLES
(107) In the Examples that follow, powders may be doped with phosphorous (P) or another element. This is indicated with the notation Cu/element symbol (?)P, such as Cu/Ni(?)P, which indicates a copper-containing particle with nickel on its surface may be doped with phosphorous. Either the copper or nickel materials (in metallic, oxide, or hydroxide form) is thought to be doped with P. Embodiments of the invention are not limited to those being doped with P or with any other element.
Example 1. Synthesis of Copper-Containing Particles with Nickel
(108) Synthesis of copper particles with a nickel-containing particulate coated surface is illustrated by the following example. 4.50 g of polyvinylpyrrolidone (PVP) MW 40,000 was dissolved in 300 mL of ethylene glycol in a 2000 mL flask. The mixture was stirred with a paddle mixer. The mixture was prepared the day before use. 144.78 g (0.5 mols) of nickel nitrate hexahydrate was added to the PVP/ethylene glycol mixture and mixed until dissolved. 381.24 g (6.0 mols) of copper powder (Example 1 was carried out separately with each of the following copper powders 1.) 10 m to 22 m, 2.) 7.5 m to 14.7 m, 3.) 1.8 m to 7.7 m, and 4.) 1.2 m to 3.2 m) was gradually added to the mixture. The mixture was continually mixed between the additions to prevent the formation of lumps. The mixture was stirred for 30 minutes with the paddle mixer. 216.24 g (2.0 mols) of sodium hypophosphite monohydrate and 150 mL of distilled water were added to the mixture. The mixture was stirred for 30 minutes with the paddle mixer.
(109) An aluminum foil lid was placed around the top of the beaker with a hole for the paddle mixer. With the beaker sitting on the hot plate, the hot plate temperature was increased to 350 C. and the mixture was heated for 40 minutes with continual stirring by the paddle mixer. The mixture was permitted to reach 90 C. After 40 minutes, the hot plate was switched off.
(110) The mixture was allowed to cool and was transferred to the centrifuge containers. The material was centrifuged at 3000 rpm for 3 minutes. The supernatant was disposed of into a waste bottle. More of the mixture was added to the centrifuge containers (or distilled water if the mixture beaker has been emptied). Stir and shake the bottle thoroughly. Centrifuging and decanting was repeated until the supernatant was clear and colorless. The solid granules were placed on a drying dish and allowed to dry overnight.
(111) The dried copper-containing particles coated with nickel oxide/hydroxide were crushed with a muller and sieved through a 325 mesh sieve.
(112) The following equation was believed to be the reaction occurring during the synthesis. The nickel oxide will deposit on the surface of the copper.
(113) Equation 1. Primary stoichiometric reaction occurring in Batch Reactor. Nickel nitrate is the rate limiting reagent.
Ni(NO.sub.3).sub.2(aq)+2NaH.sub.2PO.sub.2(aq)+2H.sub.2O.fwdarw.Ni(s)+2Na(HPO.sub.3)(aq)+3H.sub.2(g)
Example 2. Synthesis of Copper-Containing Particle with Cobalt
(114) Synthesis of copper particles with a cobalt-containing particulate coated surface is illustrated by the following example. The metal used in the coating can be changed by replacing the metal salt in Example 1 with cobalt nitrate hexahydrate. 4.50 g of polyvinylpyrrolidone (PVP) 40,000 is dissolved in 300 mL of ethylene glycol in a 2000 mL flask. The mixture was stirred with a paddle mixer. The mixture was prepared the day before use. 145.52 g (0.5 mols) of cobalt nitrate hexahydrate was added to the PVP/ethylene glycol mixture and mixed until dissolved. 381.24 g (6.0 mols) of copper powder (Example 2 was carried out separately using copper powders with a 10 m to 22 m and a 1.2 m to 3.2 m size distribution) was gradually added to the mixture. The mixture was continually mixed between the additions to prevent the formation of lumps. The mixture was stirred for 30 minutes with the paddle mixer. 216.24 g (2.0 mols) of sodium hypophosphite monohydrate and 150 mL of distilled water were added to the mixture. The mixture was stirred for 30 minutes with the paddle mixer.
(115) An aluminum foil lid is placed around the top of the beaker with a hole for the paddle mixer. With the beaker sitting on the hot plate, the hot plate temperature was increased to 350 C., and the mixture was heated for 40 minutes with continual stirring by the paddle mixer. With the hot plate set at 350 C., the mixture was permitted to reach the desired temperature of 90 C. After 40 minutes the hot plate is switched off. The mixture was allowed to cool.
(116) The mixture was transferred to the centrifuge containers and centrifuged at 3000 rpm for 3 minutes. The supernatant was disposed of into a waste bottle. Centrifuging and decanting was repeated until the supernatant was clear and colorless. The solid granules were emptied into a drying dish and dried overnight. The dried particle encrusted copper powder was crushed with a muller and sieved through a 325 mesh sieve.
Example 3. Synthesis of Copper-Containing Particles with Manganese
(117) Synthesis of copper particles with a manganese-containing particulate coated surface is illustrated by the following example. The metal used in the coating can be changed by replacing the metal salt in Example 1 with manganese nitrate tetrahydrate. 4.50 g of polyvinylpyrrolidone (PVP) 40,000 was dissolved in 300 mL of ethylene glycol in a 2000 mL flask. The mixture was stirred with a paddle mixer. The mixture was prepared the day before use. 125.51 g (0.5 mols) of manganese nitrate tetrahydrate was added to the PVP/ethylene glycol mixture and mixed until dissolved. 381.24 g (6.0 mols) of copper powder (Example 3 was carried out separately with each of a 10 m to 22 m copper powder and a 1.2 m to 3.2 m copper powder) was gradually added to the mixture. The mixture was continually mixed between the additions to prevent the formation of lumps. The mixture was stirred for 30 minutes with the paddle mixer. 216.24 g (2.0 mols) of sodium hypophosphite monohydrate and 150 mL of distilled water were added to the mixture. The mixture was stirred for 30 minutes with the paddle mixer.
(118) An aluminum foil lid was placed around the top of the beaker with a hole for the paddle mixer. With the beaker sitting on the hot plate, the hot plate temperature was increased to 350 C. and the mixture was heated for 40 minutes with continual stirring by the paddle mixer. In one example, with the hot plate set at 350 C., the mixture was permitted to reach 90 C. After 40 minutes, the hot plate was switched off, and the mixture was allowed to cool.
(119) The mixture was transferred to the centrifuge containers and centrifuged at 3000 rpm for 3 minutes. The supernatant was disposed of into a waste bottle. Decanting and centrifuging was repeated until the supernatant was clear and colorless. The solid granules were emptied into a drying dish and dried overnight.
(120) The dried powder was crushed with a muller and sieved through a 325 mesh sieve.
Example 4. Modifying the Surface Area of Copper-Containing Particles with Nickel
(121) The surface area of a coated copper powder was controlled by varying the reaction time during synthesis. The surface area of the coated powders was directly related to the time at which the reaction mixture remained above the reaction temperature. As the reactants remain above that temperature, the amount of deposited material increases which increases the surface area of the copper powders.
(122) TABLE-US-00001 TABLE 1 Temperature Time Hot Plate Peak above Surface Oxidation Switched Off Temperature 85 C. Area (XRD: Cu2O ( C.) ( C.) (min) (m.sup.2/g) (111)/Cu(111)) uncoated N/A N/A 0.60 0.001 85 90 14 0.83 0.011 90 96 27 1.59 0.001 95 100 31 2.33 0.001 100 104 32 3.14 0.000
(123) The peak temperature in the reactor and the time the reaction mixture remains above 85 C. increases the surface area of the coated copper powder. In the example, the increase is 38% at 90 C. up to a 5-fold increase at 104 C.
(124) The surface area of the coated powders is also related to the maximum temperature that the reactants experience during the reaction. The reactions using fine (1.2-3.2 microns) copper powders with an oleic acid coating are susceptible to temperature overshoot, with the copper powders with an oleic acid coating foaming during the reaction. The peak temperature was controlled by the time at which the hot plate was switched off.
(125) Gaseous by-products were trapped inside the foam layer. At peak temperatures less than 90 C., the foam layer collapsed during cooling. In addition to the powders having a lower surface area, the material also shows signs of oxidation (see Table 1). For reactions with higher peak temperatures, the foam layer remained over the reaction mixture until the material was ready to be centrifuged.
(126) When the reaction temperature exceeded 100 C., the foam layer collapsed once the reaction reached peak temperature, thus, reducing the thickness of the foam layer. This reduction in thickness, in addition to a slightly faster stirring rate, resulted in a faster cooling rate for this reaction. When the heating and cooling curves for the bench scale reactions are within a few degrees (i.e., can be directly superimposed), the measured surface area of the coated Cu (comparing 2 batches) is within 0.002 m.sup.2/g. However, even with matching heating and cooling conditions, a delay (1.5 hrs) before the material is centrifuged allows the gases to remain trapped in the foam for a longer period of time. Consequently, the surface area of the coated material was found to increase by 0.126 m.sup.2/g.
(127) Performance of pastes including unmodified copper powders with Ni/Ni/Pb nanoparticles and Sb.sub.2O.sub.3 particles are shown in
Examples 5-9. Method to Test the Durability of Devices Metallized with the Copper-Containing Paste
(128) The durability of fired solar cells was determined by monitoring the open circuit voltage (V.sub.oc) of the devices. A constant V.sub.oc indicates that the copper is not penetrating into the silicon. To speed up testing, the cells are exposed to an elevated temperature. One testing method follows the peer reviewed academic articles for copper electroplated contacts on silicon solar cells. The test thermally stresses the cell at elevated temperatures of 150 C. to 300 C. to evaluate the reliability of the cell over a much shorter period. These temperatures are above the solar cells' maximum operating temperature range of 80 C. to 90 C. One of the drawbacks of this test method is that the testing temperatures are high enough that it is possible to have chemical interactions between the materials, which would never be seen during the normal operation of a solar cell. Diffusion of copper into the junction will lead to a drop in the V.sub.oc.
Example 5. Effect of Nanoparticle Composition on Durability
(129) Three copper-containing pastes were compared. Pastes 2, 3, and 4 were formulated as shown in Table 2 (below). The nanoparticle component of the paste was composed of (i) 100% Cu/Ni/Pb nanoparticles, (ii) 50% Cu/Ni/Pb nanoparticles and 50% Ni/Ni/Pb nanoparticles, and (iii) 100% Ni/Ni/Pb nanoparticles, respectively.
(130)
(131)
(132) TABLE-US-00002 TABLE 2 Paste ID Material Paste 2 Paste 3 Paste 4 Organic Binder: 4.70 g 4.70 g 4.70 g PVP 40,000 (3.02 wt %) (3.02 wt %) (3.02 wt %) Organic Solvent: 16.68 g 16.68 g 16.68 g Propylene Glycol (10.71 wt %) (10.71 wt %) (10.71 wt %) A/B/C nanoparticles: 11.50 g 5.75 g 0 g Cu/Ni/Pb NPs (7.38 wt %) (3.69 wt %) (0 wt %) A/B/C nanoparticles: 0 g 5.75 g 11.50 g Ni/Ni/Pb NPs (0 wt %) (3.69 wt %) (7.38 wt %) Secondary Oxide: 7.16 g 7.16 g 7.16 g Sb.sub.2O.sub.3 (4.60 wt %) (4.60 wt %) (4.60 wt %) Cu/D powders: 11.58 g 11.58 g 11.58 g Cu/Ni(?)P - fine (7.43 wt %) (7.43 wt %) (7.43 wt %) copper powders (1.2-3.2 microns) Cu/D powders: 104.16 g 104.16 g 104.16 g Cu/Ni(?)P - coarse (66.86 wt %) (66.86 wt %) (66.86 wt %) copper flakes (10-20 microns)
Example 6. Effect of the Surface Modification of the Copper Containing Powders on Durability
(133) This example demonstrates an effect of the particulate coating of a copper containing powder on the PV performance of crystalline silicon cells from three different manufacturers. The copper powders used a coating consistent with Example 1. The cells prepared were thermally stressed at 200 C. for up to 110 hrs. Performance of paste 5 of Table 3 is shown in
(134) TABLE-US-00003 TABLE 3 Paste ID Material Paste 5 Organic Binder: 2.92 g PVP 40,000 (1.51 wt %) Organic Solvent: 12.69 g Propylene Glycol (6.55 wt %) A/B/C nanoparticles: 2.75 g Ni/Ni/Pb NPs (1.51 wt %) Secondary Oxide: 9.64 g Sb.sub.2O.sub.3 (4.97 wt %) Cu/D powders: 49.79 g Cu/Ni(?)P - fine (25.67 wt %) copper powders (1.2-3.2 microns) Cu/D powders: 116.05 g Cu/Ni(?)P -large (59.88 wt %) spherical copper powders (10-20 microns)
Example 7. Effect of the Relative Proportions of the Secondary Oxide Particles and Nanoparticles in the Paste on Durability
(135) This example illustrates the effect of changing the proportion of secondary oxide particles in pastes by changing the proportion of Ni/Ni/Pb nanoparticles relative to the proportion of secondary oxide particles. The weight ratio of the two particle types was kept constant at 2.50.
(136) The cells were thermally stressed at 150 C. for up to 300 hrs. The proportion of the nanoparticles and secondary oxide particles in these pastes was increased so that the volume (solids only) of copper in the paste ranged from 85.50 vol. % to 79.51 vol. % Cu. As the proportion of nanoparticles and secondary oxide particles in the paste increased, the thickness of the glassy interface between the finger and the wafer increases. The PV performance of the cells during the accelerated aging tests is maintained by maintaining the solids only portion of the paste to be greater than 80 vol. %.
(137) TABLE-US-00004 TABLE 4 Paste ID Material Paste 6 Paste 7 Paste 8 Organic Binder: 2.84 g 2.84 g 2.75 g PVP 40,000 (1.54 wt %) (1.54 wt %) (1.52 wt %) Organic Solvent: 9.24 g 10.27 g 10.47 g Propylene Glycol (5.00 wt %) (5.50 wt %) (5.81 wt %) A/B/C nanoparticles: 2.71 g 3.46 g 3.62 g Ni/Ni/Pb NPs (1.47 wt %) (1.85 wt %) (2.01 wt %) Secondary oxide: 6.79 g 8.62 g 9.01 g Sb.sub.2O.sub.3 (3.67 wt %) (4.62 wt %) (5.00 wt %) Cu/D powders: 49.01 g 48.56 g 49.74 g Cu/Ni(?)P - fine (26.50 wt %) (26.00 wt %) (25.67 wt %) copper powders (1.2-3.2 microns) Cu/D powders: 114.36 g 113.02 g 107.64 g Cu/Ni(?)P -large (61.84 wt %) (60.51 wt %) (59.78 wt %) spherical copper powders (10-20 microns) Total Vol % 82.01 80.24 79.51 Copper in Paste (solids only) (vol %)
(138) The effect of accelerated aging test at 150 C. on I-V performance of mono c-Si cells with a Passivated Emitter and Rear Cell (PERC) localized Aluminum Back Surface Field (Al BSF) metallized with the pastes is shown in
Example 8. Effect of the Relative Proportions of the Secondary Oxide in the Paste on Durability
(139) This example shows the effect of the proportion of secondary oxide particles in the paste on the performance of the cells during accelerated aging tests at 150 C. for up to 200 hrs. The weight ratio of the secondary oxide particles to nanoparticles in the pastes were varied by changing the proportion of secondary oxide particles in the paste used to print fingers. The total surface area contribution of the coated copper-containing powder used in the pastes was fixed at 0.74 m.sup.2 of coating per gram of paste. Table 5 shows the compositions of the pastes used to print the fingers. All the cells used the same bus bar paste formulation.
(140) TABLE-US-00005 TABLE 5 Paste ID Material Paste 10 Paste 11 Paste 12 Paste 13 Organic Binder: 2.79 g 2.83 g 2.88 g 2.92 g PVP 40,000 (1.53 wt %) (1.53 wt %) (1.53 wt %) (1.52 wt %) Organic Solvent: 9.54 g 9.88 g 10.18 g 10.57 g Propylene Glycol (5.23 wt %) (5.23 wt %) (5.40 wt %) (5.51 wt %) A/B/C nanoparticles: 2.75 g 2.75 g 2.75 g 2.75 g Ni/Ni/Pb NPs (1.51 wt %) (1.48 wt %) (1.46 wt %) (1.43 wt %) Secondary Oxide: 1.38 g 4.13 g 6.88 g 9.64 g Sb.sub.2O.sub.3 (0.76 wt %) (2.23 wt %) (3.65 wt %) (5.06 wt %) Cu/D powders: 49.74 g 49.74 g 49.74 g 49.74 g Cu/Ni(?)P - fine (27.29 wt %) (26.83 wt %) (26.39 wt %) (25.95 wt %) copper powders (1.2-3.2 microns) Cu/D powders: 116.05 g 116.05 g 116.05 g 116.05 g Cu/Ni(?)P -large (63.68 wt %) (62.60 wt %) (61.57 wt %) (60.55 wt %) spherical copper powders (10-20 microns) Secondary Oxide/ 0.50 1.50 2.50 3.51 NP ratio
(141)
Example 9. Effect of the Surface Area of the Copper-Containing Particle with Nickel on Durability
(142) A nickel coating on the copper-containing powders was quantified via measurement of the materials surface area. The amount of coating in the pastes was varied by changing the surface area of fine copper powders in the paste. The amount of coating on the copper powders was (Paste 11) 0.74, (Paste 14) 0.83, (Paste 15) 1.01, and (Paste 16) 1.10 m.sup.2 per gram of paste. All pastes used a secondary oxide particle to nanoparticle ratio of 1.50. Table 6 shows the compositions of the pastes used to print the fingers. All the cells used the same bus bar paste formulation.
(143) TABLE-US-00006 TABLE 6 Paste ID Material Paste 11 Paste 14 Paste 15 Paste 16 Organic Binder: 2.83 g 2.83 g 2.83 g 2.83 g PVP 40,000 (1.53 wt %) (1.52 wt %) (1.52 wt %) (1.51 wt %) Organic Solvent: 9.88 g 11.08 g 11.33 g 11.58 g Propylene Glycol (5.23 wt %) (5.94 wt %) (6.06 wt %) (6.19 wt %) A/B/C nanoparticles: 2.75 g 2.75 g 2.75 g 2.75 g Ni/Ni/Pb NPs (1.48 wt %) (1.47 wt %) (1.47 wt %) (1.47 wt %) Secondary Oxide: 4.13 g 4.13 g 4.13 g 4.13 g Sb.sub.2O.sub.3 (2.23 wt %) (2.21 wt %) (2.21 wt %) (2.21 wt %) Cu/D powders: 49.74 g 49.74 g 49.74 g 49.74 g Cu/Ni(?)P - fine (26.83 wt %) (26.66 wt %) (26.62 wt %) (26.95 wt %) copper powders (1.2-3.2 microns) Cu/D powders: 116.05 g 116.05 g 116.05 g 116.05 g Cu/Ni(?)P -large (62.60 wt %) (62.20 wt %) (62.11 wt %) (62.03 wt %) spherical copper powders (10-20 microns) Total surface area 0.74 0.83 1.01 1.10 for the copper powders per gram of paste (m.sup.2 per gram of paste)
(144)
(145)
Example 10. Effect of Copper Particles Surface Area on Etching and Fill Factor
(146) This example demonstrates the impact of the surface area of the copper-containing particles used in the paste on the etching functionality of the paste. Both the secondary oxide particles and the nanoparticles are able to etch a silicon nitride (SiN) anti-reflective coating. As the surface area contribution of the copper-containing particle is increased, the capability of the paste to etch through the SiN layer is believed to be reduced. The surface area contribution from the copper-containing particles was varied by varying the ratio of coarse to fine copper-containing powders in the paste. Table 7 lists the effect of changing this ratio on the surface area contribution of the copper-containing powders and its effect on bulk resistivity, contact resistivity, and FF of the devices. The pastes listed in Table 7 included nanoparticles with a Cu/Ni/Pb composition. It is theorized that during firing, the nanoparticles (Cu/Ni/Pb nanoparticles) and the Sb.sub.2O.sub.3 secondary oxide particles melt and coat the surface of the copper-containing particles. Consequently, the greater the surface area contribution of the copper-containing particles, the lesser the proportion of nanoparticles and secondary oxides from the secondary oxide particles that reach the cell surface. All samples were fired using the fire brick method. The fire brick method is a technique by which the sample cells were placed on a hot plate set to 300 C. for 3 mins. The sample cells were then immediately transferred to a muffle furnace set to between 750 C.-800 C. for between 1 s and 3 s. The furnace was stacked with fire bricks to retain the heat, when the furnace door was opened. The sample cells were placed directly on the surface of the fire bricks, and the door was shut for the duration of the time that the samples were in the furnace.
(147) TABLE-US-00007 TABLE 7 Ratio of fine Surface Bulk 1.2-3.2 m Area of Resistivity, Average Cu powders to Coated Cu (cm) Contact Fill coarse 10-20 in Paste (4-point Resistivity, Factor, m Cu flakes Paste ID (m.sup.2) probe method) .sub.c (cm.sup.2) FF 100:0 BTF_RD_SP27 424.80 m.sup.2 5.85 10.sup.6 0.304 0.41 50:50 BTF_RD_SP37 261.04 m.sup.2 1.54 10.sup.6 0.206 0.61 40:60 BTF_RD_SP36 228.26 m.sup.2 3.49 10.sup.6 0.048 0.62 30:70 BTF_RD_SP35 195.52 m.sup.2 2.89 10.sup.6 0.090 0.57 20:80 BTF_RD_SP34 162.75 m.sup.2 2.76 10.sup.6 0.110 0.60 10:90 Paste 2 129.99 m.sup.2 6.73 10.sup.6 0.074 0.65 0:100 BTF_RD_SP19 97.23 m.sup.2 1.61 10.sup.5 0.302 0.60 BTF_RD_SP24 4.22 10.sup.6 0.122 0.58
(148) The etching process is controlled by the amount of lead (Pb) in the paste, which may be introduced to the paste through the nanoparticles (e.g., Ni/Ni/Pb nanoparticles). The molar ratio of Ni:Pb used in the synthesis was 14:1. This amount of Pb was sufficient for a paste containing 0.83 m.sup.2 of copper per gram of paste to etch through the SiN.sub.x layer and form a working cell. Replacing the coarse copper flakes with the fine (1.2-3.2 m) copper-containing powders resulted in the paste containing 2.73 m.sup.2 of copper per gram of paste. Consequently, it was believed that the amount of lead in the paste needed to be increased to etch through the SiN.sub.x layer. One solution would be to increase the loading of the Ni/Ni/Pb nanoparticles in the paste. For this 3.3 factor increase in the surface area of copper, the molar ratio of Pb to Cu (Pb/(Pb+Cu)) needed to be increased by a factor of 12. Increasing the amount of nanoparticles in the paste was believed to be impractical due to the likelihood of a negative impact on printability of the paste. In addition, the volume of copper-containing particles in the printed contacts was reduced which impacts the resistivity, p. As a consequence, the Ni/Ni/Pb nanoparticle synthesis was modified (Ni:Pb=1.6:1) so that the nanoparticles contained a higher proportion of Pb. Subsequently the amount of nanoparticles in the paste was increased by a factor of only about 2.5.
(149) Although the pastes containing 100% of fine (1.2-3.2 m) copper powders etched through SiN.sub.x, the .sub.c of the cell remained high (0.426 cm.sup.2). The main cause of the high .sub.c was cracking in the finger effecting the measured resistances. Cracking was believed to be due to shrinkage as a result of densification. To address that issue, coarse copper flakes were used in the paste. The overall surface area of the copper in the paste was maintained at 2.73 m.sup.2 of copper per gram of paste and the quantity of nanoparticles and the secondary oxide (Sb.sub.2O.sub.3) particles was kept constant. The amount of binder and solvent in the organic vehicle was adjusted based on the proportion of solids in the paste.
(150)
Example 11. Effect of the Shape of the Copper-Containing Particles on the Rheology and Formulation of the Paste
(151) This example shows the impact of the shape of the coarse copper powders on the paste rheology, microstructure and formulation. In this example the impact of coarse spherically shaped particles was compared to the coarse flake shaped particles. The shape and packing of spherical copper-containing particles allowed the nanoparticles and the secondary oxide particles (Sb.sub.2O.sub.3) particles to melt and flow to the SiN.sub.x layer much faster than the interconnecting porous structure of the coarse copper flakes. As a result, the amount of nanoparticles and secondary oxide particles could be reduced; thus allowing the paste containing spherical particles to have a higher solids loading and a relatively lower viscosity even though the paste contained a higher proportion of fine copper containing particles. The PV performance, viscosity, and microstructure are shown in
(152) TABLE-US-00008 TABLE 8 Paste ID Paste 4 Paste 17 Ratio of Large: Small copper powders 90:10 70:30 Type of Large Copper powder Coarse Spherical Copper Copper Flakes Powders Type of Nanoparticle Ni/Ni/Pb Ni/Ni/Pb 9.2 Surface Area of Copper in paste 36.0 45.5 (m.sup.2/g of paste) Volume Fraction of copper in printed 65.0 75.2 paste after drying (vol %)
(153) Table 8 compares Paste 4 and Paste 17.
Example 12. Packing Studies for Different Shaped Copper-Containing Particles
(154) To print fingers with the lowest possible resistivity, it is desirable for the particles in the paste to pack together as closely as possible with minimum void space. The shape and size of the particles, as well as the amount of organic vehicle in the paste will determine how closely the powders will pack. Table 9 provides an estimation of the apparent tap density for the different particles used in the paste. The smaller the particle size or the larger the surface area of the particles, the lower the packing density.
(155) TABLE-US-00009 TABLE 9 Material Surface Apparent Packing Density Area (Tap) Density Factor Material (gcm.sup.3) (m.sup.2/g) (gcm.sup.3) () Cu/Ni(?)P, coarse 10-20 m 8.96 0.84 2.37 0.26 (Bean Town) Cu flakes, 325 mesh, (literature Cu > 45 m = 0.30 wt % value) 20 m < Cu < 45 m = 23.96 wt % Cu < 20 m = 75.74 wt % Cu/Ni(?)P, 10-22 m 8.96 0.29-0.73 5.40 0.60 spherical Cu powders (literature (reported (Cu112SP, Chemical Store) value) values = Cu > 45 m = 0.13 wt % 4.5-5.5) 20 m < Cu < 45 m = 86.49 wt % Cu < 20 m = 13.38 wt % Cu/Ni(?)P, 7.5-14.7 m 8.96 0.57-0.64 4.12 0.46 spherical Cu powders (610, (literature (reported ACuInternational) value) values = d.sub.10 = 7.53 m, d.sub.50 = 4.3) 9.89 m, d.sub.90 = 14.69 m Cu/Ni(?)P, 1.8-7.7 m 8.96 0.77-0.87 3.69 0.41 spherical Cu powders (603, (literature ACuInternational) value) d.sub.10 = 1.75 m, d.sub.50 = 4.03 m, d.sub.90 = 7.72 m Cu/Ni(?)P, fine 1.2-3.2 m 8.96 1.1-4.1 3.25 0.36 Cu powders (C1-2000, (literature (Still variable) 3.46 Ames Goldsmith) value) (but can be d.sub.10 = 1.2 m, d.sub.50 = increased up 1.9 m, d.sub.90 = 3.2 m to 25.62) Sb.sub.2O.sub.3 5.2 2.74 1.33 0.26 (1.1-1.8 m, Bean Town) (literature value) Ni/Ni/Pb 9.2 8.29 1.71-7.01 0.78 0.09 (assumes PbO formed (calculated (but can be and not Pb(OH).sub.2) value) increased up to 33)
(156) TABLE-US-00010 TABLE 10 coarse 10-22 m 7.5-14.7 m 1.8-7.7 m fine 1.2-3.2 m 10-20 m spherical Cu spherical Cu spherical Cu Cu powders (Bean Town) powders (Cu112SP, powders (610, powders (603, (C1-2000, Paste ID Cu flakes Chemical Store) ACuInternational) ACuInternational) Ames Goldsmith) Paste 2 90 10 Paste 41 100 Paste 18 100 Paste 19 90 10 Paste 20 80 20 Paste 17/ 70 30 Paste 5 Paste 1 37 21 12 30 Paste 21 37 21 30 12 Paste 22 37 30 21 12 Paste 23 48 33 19 Paste 24 60 43 15 Paste 25 70 30
(157) Table 10 shows the ratios of the different powders used in a selection of pastes.
(158)
(159) Table 11 (below) shows the full paste composition of Paste 5 shown in Table 10 and
(160) TABLE-US-00011 TABLE 11 Paste Id.: Paste 5 Ratio by weight of Packing Wt % Vol % the different Weight Volume Volume Space (of (solids Material Cu powders (g) (cm.sup.3) (cm.sup.3) (cm.sup.3) Paste) only) 10-22 m spherical 70 116.05 12.95 20.27 7.32 59.88 56.01 Cu powders (Cu112SP, Chemical Store) 7.5-14.7 m spherical Cu powders (610, ACuInternational) 1.8-7.7 m spherical Cu powders (603, ACuInternational) fine 1.2-3.2 m 30 49.74 5.55 8.69 3.14 25.67 24.01 Cu powders (C1-2000, Ames Goldsmith) Sb.sub.2O.sub.3 9.64 1.85 2.90 1.05 4.97 8.02 Ni/Ni/Pb 9.2 2.75 0.33 0.52 0.19 1.42 1.43 Nanoparticles PVP 40,000 2.92 2.44 1.51 10.53 Propylene Glycol 12.69 12.20 6.55 TOTAL 100 193.79 23.12 100.00 100.00 Vol. % Sb.sub.2O.sub.3 (only considering Sb.sub.2O.sub.3 and Cu) 9.11 vol. % Sb.sub.2O.sub.3/NP ratio 3.51 Cu/NP ratio 60.29 Total Cu (solids only) 80.02 vol. % Total Cu (in Paste) 85.55 wt. %
(161) TABLE-US-00012 TABLE 12 Paste Id.: Paste 1 Ratio by weight of Packing Wt. % Vol. % different Weight Volume Volume Space (of (solids Material Cu powders (g) (cm.sup.3) (cm.sup.3) (cm.sup.3) Paste) only) 10-22 m spherical 37 121.18 13.52 21.17 7.64 31.78 29.72 Cu powders (Cu112SP, Chemical Store) 7.5-14.7 m spherical 21 68.46 7.64 11.96 4.32 17.95 16.79 Cu powders (610, ACuInternational) 1.8-7.7 m spherical 12 38.68 4.32 6.76 2.44 10.14 9.49 Cu powders (603, ACuInternational) fine 1.2-3.2 m 30 97.85 10.92 17.09 6.17 25.66 24.00 Cu powders (C1-2000, Ames Goldsmith) Sb.sub.2O.sub.3 19.00 3.65 5.72 2.06 4.98 8.03 Ni/Ni/Pb 9.2 5.45 0.66 1.03 0.37 1.43 1.44 Nanoparticles PVP 40,000 5.75 4.79 1.51 10.53 Propylene Glycol 24.96 24.00 6.55 TOTAL 100 381.33 45.51 100.00 100.00 Vol. % Sb.sub.2O.sub.3 (only considering Sb.sub.2O.sub.3 and Cu) 9.12 vol. % Sb.sub.2O.sub.3/NP ratio 3.49 Cu/NP ratio 59.85 Total Cu (solids only) 80.00 vol. % Total Cu (in Paste) 85.53 wt. %
Example 13. Effect of the Organic Binder on Reducing the Oxides in the Paste and its Effect on Firing Conditions
(162) Some of the metals in the paste, specifically in the nanoparticles and in the coating on the surface of the copper-containing powders, are in the form of oxides. Introduction of oxides to the electrical contact in the cell will increase the resistivity of the fingers and will reduce the short circuit current density (J.sub.sc), the fill factor (FF), and the overall efficiency (TO) of the devices. However, the oxides are advantageous as they are simpler to synthesize and the increased surface area is useful during the etching of the antireflective (SiN) layer. Because of the impact on the overall performance of the solar cell device, these materials must be reduced during the firing. Typical techniques to reduce the oxides during the synthesis include the use of hydrogen reductants such as hydrazine, sodium borohydride, etc. Forming gases are also a common reductant, but they require specialized furnaces. Organic vehicles, such as polyvinylpyrrolidone (PVP), decompose at temperatures in excess of 300 C. That decomposition produces reducing gases capable of reducing metal oxides to metals in the paste. This allows the paste to be fired in air while simultaneously reducing selected oxides present in the paste. Exemplary pastes described herein can be used in furnaces commonly used in the PV industry.
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(164) TABLE-US-00013 TABLE 13 Material Paste 30 Organic Binder: 1.25 g PVP 40,000 (4.87 wt %) Organic Solvent: 4.70 g Ethylene Glycol (18.32 wt %) A/B/C nanoparticles: 15.00 g Cu/Ni/Pb Nanoparticles (58.48 wt %) Secondary Oxide: 2.50 g PbO.sub.2 (9.75 wt %) Graphite 0.20 g (0.78 wt %) Cu/D powders: 2.00 g Cu/Ni/Pb 10-22 m (7.80 wt %) flakes Cu powders
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(166) TABLE-US-00014 TABLE 14 Material Paste 26 Paste 27 Paste 28 Paste 29 Organic Binder: 4.70 g 3.70 g 2.70 g 1.70 g PVP 40,000 (3.02 wt %) (2.39 wt %) (1.76 wt %) (1.11 wt %) Organic Solvent: 16.68 g 16.68 g 16.68 g 16.68 g Propylene Glycol (10.71 wt %) (10.78 wt %) (10.85 wt %) (10.92 wt %) A/B/C nanoparticles: 11.50 g 11.50 g 11.50 g 11.50 g Cu/Ni/Pb Nanoparticles (7.38 wt %) (7.43 wt %) (7.48 wt %) (7.53 wt %) Secondary Oxide: 7.16 g 7.16 g 7.16 g 7.16 g Sb.sub.2O.sub.3 (4.60 wt %) (4.63 wt %) (4.66 wt %) (4.69 wt %) Cu/D powders: 115.75 g 115.75 g 115.75 g 115.75 g 10-22 m (74.30 wt %) (74.78 wt %) (75.26 wt %) (75.76 wt %) flakes Cu powders
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(169) TABLE-US-00015 TABLE 15 Paste ID Pastes Pastes Pastes Pastes 31, 32, - 31, 32, 33 - 31, 32, 33 - 31, 32, 33 - 1.5 wt % 2.0 wt % 2.5 wt % 3.0 wt % Organic Binder: 2.28 g 2.02 g 2.52 g 3.02 g PVP 40,000 (1.52 wt %) (2.01 wt %) (2.48 wt %) (2.95 wt %) Organic Solvent: 8.54 g 6.03 g 6.53 g 6.78 g Propylene Glycol** (5.71 wt %) (5.99 wt %) (6.43 wt %) (6.62 wt %) A/B/C nanoparticles: 2.15 g 1.43 g 1.43 g 1.43 g Ni/Ni/Pb Nanoparticles (1.44 wt %) (1.42 wt %) (1.41 wt %) (1.40 wt %) Secondary Oxide: 7.50 g 5.00 g 5.00 g 5.00 g Sb.sub.2O.sub.3 (5.01 wt %) (4.97 wt %) (4.92 wt %) (4.89 wt %) Cu/D powders: 38.75 g 25.83 g 25.83 g 25.83 g fine 1.2-3.2 m (25.90 wt %) (25.68 wt %) (25.43 wt %) (25.24 wt %) Cu powders* Cu/D powders: 90.42 g 60.28 g 60.28 g 60.28 g 10-22 m (61.43 wt %) (59.93 wt %) (59.34 wt %) (58.90 wt %) spherical Cu powders TOTAL 149.64 g 100.59 g 101.59 g 167.82 g (100.00 wt %) (100.00 wt %) (100.00 wt %) (100.00 wt %) *Different batches of the fine copper powders were used in order to change the surface area of the fine copper powders to produce pastes 31, 32, and 33. **The amount of propylene glycol would have been adjusted in order to produce pastes with different viscosities.
Example 14. Effect of Secondary Oxide Particles and Nanoparticles on the Electrical Properties of the Paste
(170) In this example, the impact of the secondary oxide particles (e.g., Sb.sub.2O.sub.3) and the nanoparticles (e.g., Cu/Ni/Pb) on the bulk resistivity (p) and contact resistivity (.sub.c) is demonstrated. Antimony, Sb, is a known n-type dopant for silicon and was introduced as a route to possibly form a n.sup.+ region below the contact, and hence lower the contact resistivity. In this example, a commercially available antimony oxide with a particle size range of 1.1-1.8 m was used. Antimony oxide has a low melting point (656 C.) and lies just below nickel oxide, NiO on the Ellingham diagram. As a result, it requires a similar energy input, to be reduced to its elemental form. Additionally, its position means that antimony can reduce the oxides of the other elements (Ni, Cu, Pb) in the paste, resulting in a more conductive contact. A factorial design of experiments approach was used to introduce antimony oxide to the paste. In this case, the oxide was considered to be part of the glass frit system (in addition to the Cu/Ni/Pb nanoparticles) which chemically reacts with the SiN layer. The amount of glass frits in the paste was kept constant and the ratio of the nanoparticles to secondary oxide particles was varied in 2.sup.2 design matrix using predetermined high or low values for the two components. This resulted in four possible paste combinations, that were mixed, screen printed on mono crystalline silicon solar cells, and fired at 690 C. for 3 s in air (using the fire brick method). The effect of using high and low amounts of the secondary oxide particles in the paste on the bulk resistivity, contact resistance, as well as the devices PV performances was investigated. Tables 16, 17, and 18 (below) list the paste compositions for the design matrix.
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(172) The introduction of the secondary oxides to the paste had a large effect in improving both the bulk resistivity and contact resistivity; and hence the overall PV performance of the pastes. SEM images (not shown) of the cross sections showed improved connectivity between the copper particles, but still had a large degree of porosity remaining in the films.
(173) TABLE-US-00016 TABLE 16 A: Cu/Ni/Pb nanoparticles B: Antimony Oxide Low Value () 9.0 g 5.6 g High Value (+) 12.6 g 8.4 g
(174) TABLE-US-00017 TABLE 17 Paste ID A: Cu/Ni/Pb nanoparticles B: Antimony Oxide BTF_RD_SP5 0.6190 0.3810 BTF_RD_SP6 0.6923 0.3077 BTF_RD_SP7 0.5200 0.4800 BTF_RD_SP8 0.6000 0.4000
(175) TABLE-US-00018 TABLE 18 Material Paste 34 Paste 35 Paste 36 Paste 37 Organic Binder: 4.46 g 5.56 g 5.43 g 6.41 g PVP 40,000 (3.60 wt %) (3.60 wt %) (3.60 wt %) (3.60 wt %) Organic Solvent: 14.28 17.80 17.37 20.53 Propylene Glycol (11.52 wt %) (11.52 wt %) (11.52 wt %) (11.52 A/B/C nanoparticles: 9.00 g 12.60 g 9.00 g 12.60 g Cu/Ni/Pb Nanoparticles (7.26 wt %) (8.16 wt %) (5.97 wt %) (7.07 wt %) Secondary Oxide: 5.60 g 5.60 g 8.40 g 8.40 g Sb.sub.2O.sub.3 (4.52 wt %) (3.62 wt %) (5.57 wt %) (4.71 wt %) Cu/D powders: 90.59 g 112.92 g 110.20 g 130.30 g 10-22 m (73.09 wt %) (73.09 wt %) (73.09 wt %) (73.09 flakes Cu powders A/B combination used Low/Low High/Low Low/High High/High in 2.sup.2 factorial design
Example 15. Secondary Oxide Particles Acting as a Liquid Sintering Agent
(176) It is desirable for the fingers to have a denser microstructure to lower the resistivity of the contacts, and hence the series resistance of the devices. The secondary oxide particles, such as antimony oxide particles, acts as a liquid sintering aid due to their relatively low melting point (for Sb.sub.2O.sub.3, melting temperature is 656 C.). A higher proportion of the secondary oxide particles in the paste results in a lower sintering temperature and/or faster sintering process, as well as densification of the contact.
(177) Table 19 shows the composition of a Paste 38 using a 70:30 blend of coarse to fine copper-containing particles (a similar blend to Paste 5). The amount of secondary oxide particles was increased to about 22 wt. %.
(178) TABLE-US-00019 TABLE 19 Paste Id.: Paste 38 Ratio by weight of Packing Wt % Vol % the different Weight Volume Volume Space (of (solids Material Cu powders (g) (cm.sup.3) (cm.sup.3) (cm.sup.3) Paste) only) 10-22 m spherical 70 116.05 12.95 20.27 7.32 47.00 38.86 Cu powders (Cu112SP, Chemical Store) 7.5-14.7 m spherical Cu powders (610, ACuInternational) 1.8-7.7 m spherical Cu powders (603, ACuInternational) fine 1.2-3.2 m 30 49.74 5.55 8.69 3.14 20.14 16.66 Cu powders (C1-2000, Ames Goldsmith) Sb.sub.2O.sub.3 55.00 10.58 16.55 5.98 22.27 31.74 Ni/Ni/Pb 9.2 2.75 0.33 0.52 0.19 1.11 1.00 Nanoparticles PVP 40,000 4.70 3.92 1.90 11.75 Propylene Glycol 18.68 17.96 7.57 TOTAL 100 246.92 33.33 100.00 100.00
(179) To densify the contacts further, without causing delamination and voids in the microstructure, the copper-containing particles need to pack together as closely as possible and have a larger volume percentage in the contact. To improve the density of the contacts, packing density studies of the copper-containing particles were carried out using the spherical powders shown in Table 8 (above) of Example 11. Calculations were carried out assuming that the powders had a maximum packing factor of 0.639, and that the smaller spherical powders occupy the free space between the large copper particles. Once this value was calculated, the finest (1.2-3.2 m) copper powder was added at 30 wt. % of the copper powder in the paste. These calculations resulted in the following blend of copper-containing particles: 37 wt. % (10-22 m), 21 wt. % (7.5-14.7 m), 12 wt. % (1.8-7.7 m), and 30 wt. % (1.2-3.2 m).
(180) The effect of the secondary oxide particles as a liquid sintering aid were studied by increasing the volume percentage of the oxide particles in the pastes containing a blend of four copper-containing powders with different particle sizes. The compositions of the pastes are shown in Table 20.
(181) TABLE-US-00020 TABLE 20 Material Paste 1 Paste 40 Paste 39 Organic Binder: 5.75 g 5.99 g 6.27 g PVP 40,000 (1.51 wt %) (1.51 wt %) (1.51 wt %) Organic Solvent: 24.96 26.01 27.21 Propylene Glycol (6.55 wt %) (6.55 wt %) (6.55 wt %) A/B/C nanoparticles: 5.45 g 5.45 g 5.45 g Ni/Ni/Pb 9.2 Nanoparticles (1.43 wt %) (1.37 wt %) (1.31 wt %) Secondary Oxide: 19.00 g 33.72 g 50.55 g Sb.sub.2O.sub.3 (4.98 wt %) (8.49 wt %) (12.16 wt %) Cu/S powders: 97.85 g 97.85 g 97.85 g fine 1.2-3.2 m (25.66 wt %) (24.63 wt %) (23.54 wt %) Cu powders Cu/D powders: 38.68 g 38.68 g 38.68 g 1.8-7.7 m (10.14 wt %) (8.89 wt %) (9.31 wt %) spherical Cu powders Cu/D powders: 68.46 g 68.46 g 68.46 g 7.5-14.7 m (17.95 wt %) (15.74 wt %) (16.47 wt %) spherical Cu powders Cu/D powders: 121.18 g 121.18 g 121.18 g 10-22 m (31.78 wt %) (30.50 wt %) (29.15 wt %) spherical Cu powders
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(183) In
Example 16. Effect of the Secondary Oxide Particles and Nanoparticles on the Etching Function of the Paste and the Firing Conditions
(184) In this example, the role the secondary oxide particles and nanoparticles play on the etching functions of the paste is demonstrated. The total surface area contribution of the copper-containing particles used in the pastes was fixed at 0.74 m.sup.2 per gram of paste. The pastes were printed and fired using the following setup.
(185) The cells were fired in a semi-automated furnace. The system consists of a meter long quartz tube with a volume of approximately 2.4 L. The furnace was built with a low temperature zone (zone 1) and a high temperature zone (zone 2) along the length of the quartz tube. Compressed air with a flow rate of 1 L/min was introduced from the start of zone 1. The cells were placed on a nichrome mesh stage, which has been automated using a stepper motor and Arduino software program, for control of the cell's movement speed. The speed S has an inverse relationship to the time T it takes the cell to travel from zone 1 to zone 2 according to (T=38.74S.sup.1). Zone 1 and zone 2 set point temperatures were 150 C. and 753 C., respectively. In a firing procedure, the cell was placed on the nichrome mesh and moved into zone 1 at speed, S, where it was held for 180 s. The cell was then moved in to zone 2 at speed, S and held in zone 2 for time, t.sub.hold. The stage was then moved back to the start using a speed of 32.4 cm/s. This returned the stage to the start in less than 2 s. The cell was subsequently removed from the stage and rapidly cooled to room temperature by placing it on a metal plate.
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(187) TABLE-US-00021 TABLE 21 Paste ID Material Paste 9 Paste 10 Paste 11 Paste 12 Paste 13 Organic Binder: 2.76 g 2.79 g 2.83 g 2.88 g 2.92 g PVP 40,000 (1.53 wt %) (1.53 wt %) (1.53 wt %) (1.53 wt %) (1.52 wt %) Organic Solvent: 9.47 g 9.54 g 9.88 g 10.18 g 10.57 g Propylene Glycol (5.24 wt %) (5.23 wt %) (5.23 wt %) (5.40 wt %) (5.51 wt %) A/B/C nanoparticles: 2.75 g 2.75 g 2.75 g 2.75 g 2.75 g Ni/Ni/Pb NPs (1.52 wt %) (1.51 wt %) (1.48 wt %) (1.46 wt %) (1.43 wt %) Secondary Oxide: 0 g 1.38 g 4.13 g 6.88 g 9.64 g Sb.sub.2O.sub.3 (0 wt %) (0.76 wt %) (2.23 wt %) (3.65 wt %) (5.06 wt %) Cu/D powders: 49.74 g 49.74 g 49.74 g 49.74 g 49.74 g Cu/Ni(?)P - fine (27.52 wt %) (27.29 wt %) (26.83 wt %) (26.39 wt %) (25.95 wt %) copper powders (1.2-3.2 microns) Cu/D powders: 116.05 g 116.05 g 116.05 g 116.05 g 116.05 g Cu/Ni(?)P -large (64.20 wt %) (63.68 wt %) (62.60 wt %) (61.57 wt %) (60.55 wt %) spherical copper powders (10-20 microns) Total vol % Cu 87.53 vol % 86.37 vol % 84.15 vol % 82.03 vol % 80.02 vol % (solids only) Copper Containing 60.29 60.29 60.29 60.29 60.29 Particles/ NP ratio (by weight) Secondary Oxide/ 0.50 1.50 2.50 3.51 NP ratio (by weight) Copper Containing 120.14 40.14 24.10 17.20 Particles to Secondary Oxide ratio (by weight)
Example 17. Effect of the Surface Area of the Copper-Containing Particles with Nickel on the Etching Functions of the Paste and Firing Conditions
(188) This example shows the effect the copper-containing particles have on the etching and firing conditions of the paste. Table 6 shows the compositions of the pastes used in this example. All the pastes used a secondary oxide particles of antimony oxide to nanoparticle (Ni/Ni/Pb) ratio of 1.50. The proportion of nickel nanoparticles in the coating surrounding the copper particles is quantified via the surface area of the coated copper powders.
(189) To vary the proportions of nickel nanoparticles in the paste, the surface area of the fine copper-containing particles in the pastes were varied. This was done by blending different batches of copper powders with surface areas of 1.13, 2.61 and 25.63 m.sup.2/g to produce pastes containing 0.74 (Paste 11), 0.83 (Paste 14), 1.01 (Paste 15) and 1.10 (Paste 16) m.sup.2 of copper-containing particles per gram of paste.
(190) Samples were fired using the firing process described in Example 16. The cells, were fired using zone 2 temperatures of 753 C., 778 C., and 803 C. with a stage speed of 1.43 cm/s. Table 22 (below) shows the time for which the cells were held in zone 2 to etch through the SiN layer and make a contact to produce FF>0.6. As shown, the time required increased as the surface area of the copper-containing particles was increased. When a highest zone 2 temperature (i.e., 803 C.) was used, the nickel particles on the copper particles no longer assisted in slowing down the etching process, and actually resulted in the cells being overfired.
(191) TABLE-US-00022 TABLE 22 Surface Area contribution Hold Time Zone 2 (s) from the copper containing Zone 2 Temperature ( C.) particles (m.sup.2 per gram of paste) 753 C. 778 C. 803 C. 0.74 m.sup.2 per gram of paste 3 s 2 s 0 s 0.83 m.sup.2 per gram of paste 4 s 3 s 1 s 1.01 m.sup.2 per gram of paste 6 s 3 s 2 s (overfired) 1.10 m.sup.2 per gram of paste 6 s 4 s 2 s (overfired)
Example 18. Effect of Firing Duration on Performance During Accelerated Aging Tests
(192) This example shows the effect the firing conditions can have on the durability of the cells under accelerated aging conditions. The paste was used to metallize mono crystalline silicon solar cells with a full Al BSF rear contact. The paste (Paste 15) shown in Table 6 used a total of 1.01 m.sup.2 of copper containing particles with nickel per gram of paste. The cells were fired using the procedure described in Example 16 and a stage speed of 1.43 cm/s and zone 2 temperature of 753 C. The time the cells were held in zone 2 was varied.
Example 19: Secondary Oxide to Nanoparticle Ratio
(193) The secondary oxide particles, nanoparticles, and particles from the coated copper-containing particles in the paste form a liquid sintering aid during the firing process, resulting in the fingers forming a denser contact. The nanoparticles contain lead oxides/hydroxides, which when combined with the other materials lowers the melting point of the mixture. As the organic material burns off, it can expel droplets of the liquid sintering aid across the surface of the cell. These droplets can create an undesirable cosmetic blemish to the surface of the cell. The surface blemish is thought to impede the number of photons reaching the active region of the cell, and thus reduce power output. By changing the ratio of the secondary oxide to nanoparticle ratio, the amount of droplet appearing on the surface can be controlled (
(194) TABLE-US-00023 TABLE 23 Paste ID Material Paste 42 Paste 43 Paste 44 Paste 45 Paste 46 Paste 47 Organic Binder: 4.26 g 4.28 g 4.31 g 4.34 g 4.36 g 4.39 g PVP 40,000 (2.06 wt %) (2.06 wt %) (2.05 wt %) (2.05 wt %) (2.05 wt %) (2.04 wt %) Organic Solvent: 18.59 g 18.71 g 19.35 g 20.24 g Dipropylene Glycol (8.97 wt %) (8.98 wt %) (9.42 wt %) (9.43 wt %) Organic Solvent: 19.85 g 20.09 g Tripropylene Glycol (9.37 wt %) (9.42 wt %) A/B/C nanoparticles: 0 g 1.11 g 2.22 g 3.33 g 4.44 g 5.55 g Ni/Ni/Pb NPs (0 wt %) (0.53 wt %) (1.06 wt %) (1.57 wt %) (2.08 wt %) (2.59 wt %) (higher Pb content) Secondary Oxide: 18.59 g 18.59 g 18.59 g 18.59 g 18.59 g 18.59 g Sb.sub.2O.sub.3 (8.97 wt %) (8.92 wt %) (8.84 wt %) (8.77 wt %) (8.72 wt %) (8.66 wt %) Cu/D powders: 165.79 g 165.79 g 165.79 g 165.79 g 165.79 g 16.79 g Cu/Ni(?)P - fine (80.00 wt %) (79.52 wt %) (78.85 wt %) (78.24 wt %) (77.74 wt %) (77.27 wt %) copper powders (1.2-3.2 microns) Secondary Oxide to 16.75 8.37 5.58 4.19 3.35 Nanoparticle ratio Total surface area 0.51 0.51 0.51 0.51 0.51 0.60 for the copper powders per gram of paste (m.sup.2 per gram of paste)
(195) The amount of paste used to print the fingers directly impacts the maximum secondary oxide particle to nanoparticle ratio that can be used; with fingers using less paste being able to tolerate a relatively smaller ratio, before this cosmetic blemish becomes an issue. However, pastes using a higher ratio, require a higher proportion of the overall amount of glass frits (i.e., combined secondary oxides and nanoparticles), as the secondary oxide is a less aggressive etchant than the nanoparticles. As the finger width (W) dimension decreases (see
Example 20: Effect of Finger Dimension on Paste Composition
(196) This example illustrates the impact of finger dimensions on the composition of the paste. The finger dimension can be reduced by reducing the number of prints, leading to a reduction in the height of the fingers. The height as well as the width of the fingers can also be changed by reducing the dimensions of the pattern, mesh and emulsion thickness used by the screen during the screen-printing process.
(197) Reducing the height of the fingers reduces the amount of paste on the cell. This reduces the proportion of nanoparticles and secondary oxide particles available per unit area. At a certain point, the amount of these materials is insufficient to etch through the anti-reflective coating. The etching rate of the combined nanoparticles and secondary oxide particles is lowered by the particles coated on the copper-containing particles. The more particles in the paste, the more nanoparticles and secondary oxide particles are required. However, as the proportion of these materials are increased, the bulk resistivity gradually increases. The quantity of particles in the paste is defined by the surface area of the coated copper-containing particles.
(198) The amount of power generated by a solar cell is dependent on the number of photons of light reaching the cell. Therefore, the larger the area covered by the electrical conductors, the less photons reach the cell and the lower the power output. In order reduce the dimensions of the fingers, and thus the amount of paste used to print the fingers without impacting the performance of the devices, the surface area for the copper containing particle per gram of paste needed to be reduced. To reduce the finger width, a pattern opening size on a screen is reduced. Since, the theoretical maximum particle size should be at least 2.5 times smaller than the mesh opening, the average particle size also needed to be reduced. The copper-containing particle has the largest particles in the paste. Consequently, reducing the particle size distribution range of this material naturally leads to an increase in the amount of coating in the paste. When the particle size range of coarse copper-containing particles in the paste was lowered from a particle size range of 10-22 m to 7.5-14.7 m, the proportion of binder in the paste was increased, as the larger surface area made the powders more susceptible to oxidation during the firing process. The pastes were printed on poly crystalline silicon solar cells and fired using the same semi-automated furnace described in Example 16.
(199) TABLE-US-00024 TABLE 24 Paste ID Material Paste 48 Paste 49 Paste 50 Paste 51 Paste 52 Paste 53 Organic Binder: 4.48 g 4.65 g 4.82 g 4.99 g 4.65 g 4.82 g PVP 40,000 (2.37 wt %) (2.37 wt %) (2.37 wt %) (2.37 wt %) (2.37 wt %) (2.37 wt %) Organic Solvent: 12.46 g 12.94 g 13.41 g 13.89 g 12.94 g 13.41 g Propylene Glycol (6.58 wt %) (6.58 wt %) (6.58 wt %) (6.58 wt %) (6.58 wt %) (6.58 wt %) A/B/C nanoparticles: 0.37 g 0.74 g 1.11 g 1.48 g 0.74 g 1.11 g Ni/Ni/Pb NPs (0.20 wt %) (0.38 wt %) (0.54 wt %) (0.70 wt %) (0.38 wt %) (0.54 wt %) Secondary Oxide: 6.20 g 12.39 g 18.59 g 24.78 g 12.39 g 18.59 g Sb.sub.2O.sub.3 (3.28 wt %) (6.31 wt %) (9.31 wt %) (11.75 wt %) (6.31 wt %) (9.13 wt %) Cu/D powders: 49.74 g 49.74 g 49.74 g 49.74 g 49.74 g 49.74 g Cu/Ni(?)P - fine (26.28 wt %) (25.31 wt %) (24.42 wt %) (23.58 wt %) (25.31 wt %) (24.42 wt %) copper powders (1.2-3.2 microns) Cu/D powders: 116.05 g 116.05 g 116.05 g 116.05 g 116.05 g 116.05 g Cu/Ni(?)P - spherical (61.30 wt %) (59.06 wt %) (56.96 wt %) (55.02 wt %) (59.06 wt %) (56.96 wt %) copper powders (7.5-14.7 microns) Secondary Oxide to 16.76 16.74 16.75 16.74 16.74 16.75 Nanoparticle ratio Total amount of 3.48 wt % 6.69 wt % 9.67 wt % 12.45 wt % 6.69 wt % 9.67 wt % glass frits Total surface area 0.80 0.78 0.75 0.72 0.57 0.55 for the copper powders per gram of paste (m.sup.2 per gram of paste)
(200) The screen-printing process was reduced to a single print by reducing the surface area of the fine copper powders down to 1.15 m.sup.2/g; thus resulting in a total surface area contribution from the copper-containing particle of 0.40 m.sup.2 per gram of paste. Full sized poly c-Si cells were printed using pastes with a 0.40 m.sup.2 per gram of paste, and a secondary oxide to nanoparticle ratio of between 16.70 and 16.75. The proportion of binder in the pastes was increased from 1.69 to 2.32 wt. %. The cells were fired in a TP Solar Model M-225 (Neutrino Series) IR belt furnace. The furnace contained six heated zones and a cooling zone approximately 30% longer that the combined length of the heated sections. The zones are built such that the front and rear of the cell were heated simultaneously using IR heat lamps. The temperatures for zones 1 through 5 were 620, 460, 440, 450 and 520 C., respectively. Zone 6 was varied from 665 C. to 690 C. The belt speed was varied from 225-400 in/min. The cells were sectioned into 44 cm2 samples in order to measure the PV performance.
(201) TABLE-US-00025 TABLE 25 Paste ID Material Paste 54 Paste 55 Organic Binder: 9.65 g 13.81 g PVP 40,000 (1.69 wt %) (2.32 wt %) Organic Solvent: 44.97 g 50.90 g Propylene Glycol (7.88 wt %) (8.54 wt %) A/B/C nanoparticles: 3.09 g 3.18 g Ni/Ni/Pb NPs (0.54 wt %) (0.53 wt %) Secondary Oxide: 51.60 g 53.26 g Sb.sub.2O.sub.3 (9.05 wt %) (8.93 wt %) Cu/D powders: 138.35 g 142.51 g Cu/Ni(?)P - fine (24.25 wt %) (23.91 wt %) copper powders (1.2-3.2 microns) Cu/D powders: 322.78 g 332.49 g Cu/Ni(?)P - spherical (56.58 wt %) (55.77 wt %) copper powders (7.5-14.7 microns) Secondary Oxide to 16.70 16.75 Nanoparticle ratio Total amount of 9.59 wt % 9.46 wt % glass frits Total surface area 0.41 0.41 for the copper powders per gram of paste (m.sup.2 per gram of paste)
(202) The width and mass of paste used to print the fingers can be decreased further by reducing the size of the screen opening. To accomplish this goal only copper-containing particles with a size range of 1.20 to 3.20 m were used in the paste.
(203) Lowering the surface area contribution of the smaller copper-containing particle 46 further can limit the belt speed at which the cells can be fired.
(204) TABLE-US-00026 TABLE 26 Paste ID Material Paste 56 Paste 57 Paste 58 Organic Binder: 7.05 g 7.05 g 7.05 g PVP 8,000 (3.28 wt %) (3.29 wt %) (3.29 wt %) Organic Solvent: 17.99 g 16.99 g 16.99 g Dipropylene Glycol (8.37 wt %) (7.94 wt %) (7.94 wt %) A/B/C nanoparticles: 5.55 g 5.55 g 5.55 g Ni/Ni/Pb NPs (2.58 wt %) (2.59 wt %) (2.59 wt %) (higher Pb content) Secondary Oxide: 18.59 g 18.59 g 18.59 g Sb.sub.2O.sub.3 (8.65 wt %) (8.69 wt %) (8.69 wt %) Cu/D powders: 165.79 g 165.79 g 165.79 g Cu/Ni(?)P - fine (77.12 wt %) (77.48 wt %) (77.48 wt %) copper powders (1.2-3.2 microns) Secondary Oxide to 3.35 3.35 3.35 Nanoparticle ratio Total amount of 11.23 wt % 11.28 wt % 11.28 wt % glass frits Surface Area of 0.66 m.sup.2/g 0.73 m.sup.2/g 0.88 m.sup.2/g Cu/Ni(?)P - fine copper powders (1.2-3.2 microns) Total surface area 0.51 0.57 0.68 for the copper powders per gram of paste (m.sup.2 per gram of paste)
(205) The changes to the pastes composition, surface area contribution and resulting amount of paste required to generate devices with a fill factor>0.6 are summarized in Table 27.
(206) TABLE-US-00027 TABLE 27 Ratio of the different sized copper Average mass of paste used containing powders used in the paste and No. of fingers to print fingers on cell their corresponding surface area (m.sup.2/g) printed and (Values in italic are calculated) 10-22 m 7.5-14.7 m width of screen Full Sized 1/16 spherical Cu spherical Cu Material/ pattern used to No. of Cell ~15.8 cell ~4 powders (Cu112SP, powders (610, Paste ID print the fingers Prints 15.8 cm.sup.2 4 cm.sup.2 Chemical Store) ACuInternational) Paste 5 74 3 10 64 mg 70 200 m 24 mg 0.25 m.sup.2/g Paste 5 74 2 90 56 mg 70 200 m 1 mg 0.25 m.sup.2/g Paste 53 74 3 36 23 mg 70 100 m 0 mg 0.33 m.sup.2/g Paste 62 74 2 30 19 mg 100 m 5 mg Paste 54 74 1 23 15 mg 70 100 m 3 mg 0.24 m.sup.2/g Paste 55 74 1 19 12 mg 70 100 m 1 mg 0.24 m.sup.2/g Paste 55 116 1 22 14 mg 70 83 m 4 mg 0.22 m.sup.2/g Paste 58 116 1 17 11 mg 70 83 m 5 mg 0.23 m.sup.2/g Paste 56 88 1 94 mg 6 mg 60 m Ratio of Ratio of the different sized copper Total surface secondary oxide containing powders used in the paste and area contribution particles to their corresponding surface area (m.sup.2/g) of the copper nanoparticles 1.8-7.7 m Fine 1.2-3.2 m containing and total amount Amount of Material/ Spherical (603, Cu powders, powders (m.sup.2 per of glass frits binder in Paste ID ACuInternational) Ames Goldsmith) gram of paste) in the paste the paste Paste 5 30 0.75 3.50 1.54 wt % 2.31 m.sup.2/g 6.49 wt % Paste 5 30 0.75 3.50 1.54 wt % 2.31 m.sup.2/g 6.49 wt % Paste 53 30 0.58 16.91 2.37 wt % 1.59 m.sup.2/g 9.67 wt % Paste 62 70 30 0.89 16.68 2.36 wt % 0.59 m.sup.2/g 1.59 m.sup.2/g 9.59 wt % Paste 54 30 0.41 16.70 1.69 wt % 1.15 m.sup.2/g 9.59 wt % Paste 55 30 0.41 16.75 2.32 wt % 1.15 m.sup.2/g 9.46 wt % Paste 55 30 0.40 16.75 2.32 wt % 1.16 m.sup.2/g 9.46 wt % Paste 58 30 0.40 8.40 2.68 wt % 1.12 m.sup.2/g 10.03 wt % Paste 56 100 0.51 3.35 3.28 wt % 0.66 m.sup.2/g 11.23 wt %
Example 21: Controlling the Etching Depth and the Impact of the Binder on Durability
(207)
Example 22: Impact of the Composition of the Nanoparticles
(208) The nanoparticles and secondary oxide particles provide the etching functionality of the paste. The rate of etching can be varied by changing the ratio of these two materials in the paste. It can also be changed by changing the composition of the nanoparticles. The aggressiveness of nanoparticles can be controlled by changing the ratio of the different elements during the synthesis of the nanoparticles, as described above. In this example the molar ratio of nickel (Ni) and lead (Pb) in the Ni/Ni/Pb nanoparticles was decreased from 1.52 to 0.52, during the synthesis. Pastes were made using these two nanoparticle compositions and were printed on mono crystalline silicon PERC cells. The cells were fired in the three zone semi-automated firing system described in Example 20. The temperatures in the zone's 1, 2 and 3 were 650, 450 and 925 C., respectively.
(209) TABLE-US-00028 TABLE 28 Paste ID Material Paste 55 Paste 56 Paste 57 Paste 58 Organic Binder: (2.31 wt %) (2.35 wt %) (2.67 wt %) (2.68 wt %) PVP 40,000 Organic Solvent: (9.18 wt %) (7.53 wt %) (7.81 wt %) (7.34 wt %) Propylene Glycol A/B/C nanoparticles: (0.53 wt %) (1.06 wt %) (1.06 wt %) Ni/Ni/Pb NPs A/B/C nanoparticles: (0.53 wt %) Ni/Ni/Pb NPs (higher Pb content) Secondary Oxide: (8.87 wt %) (9.03 wt %) (8.92 wt %) (8.96 wt %) Sb.sub.2O.sub.3 Cu/D powders: (23.74 wt %) (24.17 wt %) (23.86 wt %) (23.98 wt %) Cu/Ni(?)P - fine copper powders (1.2-3.2 microns) Cu/D powders: (55.39 wt %) (55.77 wt %) (55.69 wt %) (55.97 wt %) Cu/Ni(?)P - spherical copper powders (7.5-14.7 microns) Secondary Oxide to 16.89 16.89 8.45 8.45 Nanoparticle ratio Total amount of 9.40 wt % 9.56 wt % 9.98 wt % 10.02 wt % glass frits Total surface area 0.40 0.41 0.41 0.41 for the copper powders per gram of paste (m.sup.2 per gram of paste)
Example 23: Impact of the Size of the Copper Containing Particles on Firing Conditions
(210) Reduction to the finger dimensions, requires not only changes to the paste composition, but also changes to the firing conditions. The smaller the size of the copper-containing particles, the more susceptible the particles are to oxidation. Increasing the proportion of binder in the paste, as well as the amount of etchants in the paste aids in reducing any oxides, as well as those formed during firing. However, both components can only be increased to a certain level before they impact the devices performance. Therefore, changes to the firing conditions were required. The cells were fired using the three zone semi-automated firing system described in Example 20. In the first two zones most of the binder is burnt off, but with smaller finger dimensions, the proportion of binder present is already smaller. Consequently, the temperature in the first zone was reduced, to account for this impact.
(211) TABLE-US-00029 TABLE 29 Paste ID Material Paste 59 Paste 60 Paste 61 Organic Binder: 4.52 g 5.20 g 5.88 g PVP 40,000 (2.04 wt %) (2.35 wt %) (2.64 wt %) Organic Solvent: 20.80 g 20.80 g 21.11 g Propylene Glycol (9.42 wt %) (9.39 wt %) (9.49 wt %) A/B/C nanoparticles: 11.10 g 11.10 g 11.10 g Ni/Ni/Pb NPs (5.03 wt %) (5.01 wt %) (4.99 wt %) (higher Pb content) Secondary Oxide: 18.59 g 18.59 g 18.59 g Sb.sub.2O.sub.3 (8.42 wt %) (8.39 wt %) (8.36 wt %) Cu/D powders: 165.79 g 165.79 g 165.79 g Cu/Ni(?)P - fine (75.09 wt %) (74.86 wt %) (74.52 wt %) copper powders (1.2-3.2 microns) Secondary Oxide to 1.67 1.67 1.67 Nanoparticle ratio Total amount of 13.45 wt% 13.40 wt% 13.35 wt% glass frits Surface Area of 0.78 m.sup.2/g 0.78 m.sup.2/g 0.78 m.sup.2/g Cu/Ni(?)P - fine copper powders (1.2-3.2 microns) Total surface area 0.58 0.58 0.58 for the copper powders per gram of paste (m.sup.2 per gram of paste)
Example 24: Increasing the Screen Life of the Pastes for Smaller Finger Dimensions
(212) The reduction in the size of the particles in the paste can lead to a reduced screen life for the paste, as the increased surface area increases the rate at which the paste dries. The screen life can be extended by directly substituting the solvent used in the medium with a solvent with a lower evaporation rate. Table 30 shows the physical properties of the solvents that have been tested in the paste.
(213) TABLE-US-00030 TABLE 30 Boiling Evaporation Point Viscosity Rate (n-Butyl Solvent Name ( C.) (cP) Acetate = 1) Propylene Glycol 187.4 48.6 0.0157 Dipropylene Glycol 232.2 75.0 0.00155 Tripropylene Glycol 265.1 57.2 0.000214 Butyl Carbitol (Diethylene 230 4.9 0.004 Glycol Monbutyl Ether) Texanol Ester Alcohol 255-260.5 13.5 0.002 (2,2,4-Trimethyl 1.3- pentanediol monoisobuyrate)
Example 25: Finger Dimension
(214) Table 31 shows the average dimensions of a high viscosity paste (Paste 53) printed through a 300 mesh screen with a 20 m emulsion over mesh (EOM) coating and 100 m wide finger opening. The cells were fired using a TP Solar six zone IR belt furnace with zone 1, 2, 3, 4, 5 and 6 temperatures of 620, 460, 440, 450, 520 and 840 C., respectively. The average width and height of the fingers for the unfired and fired fingers were measured from cross sectional images using an optical microscope and scanning electron microscope, respectively.
(215) TABLE-US-00031 TABLE 31 Average mass of Screen Average Average Average Average paste used to print opening width of height of width of height of No. of fingers on cell 1/16 for unfired unfired fired fired No. of fingers cell ~4 4 cm.sup.2 fingers fingers fingers fingers fingers Paste ID prints (mg) (mg) (m) (m) (m) (m) (m) Paste 53 3 18 23 100 115 82 91.8 55.4
(216) The printed width (see e.g., W.sub.2 in
(217) TABLE-US-00032 TABLE 32 Paste ID Material Paste 63 Organic Binder: 8.36 g PVP 8,000 (3.86 wt %) Organic Solvent: 17.99 g Dipropylene Glycol (8.32 wt %) A/B/C nanoparticles: 5.55 g Ni/Ni/Pb NPs (2.57 wt %) (higher Pb content) Secondary Oxide: 18.59 g Sb.sub.2O.sub.3 (8.60 wt %) Cu/D powders: 165.79 g Cu/Ni(?)P - fine (76.66 wt %) copper powders (1.2-3.2 microns) Secondary Oxide to 3.35 Nanoparticle ratio Total amount of 11.17 wt% glass frits Surface Area of 0.67 m.sup.2/g Cu/Ni(?)P - fine copper powders (1.2-3.2 microns) Total surface area 0.51 for the copper powders per gram of paste (m.sup.2 per gram of paste)
(218) While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in some detail, it is not the intention of the inventors to restrict or in any way limit the scope of the appended claims to such detail. Thus, additional advantages and modifications will readily appear to those of ordinary skill in the art. The various features of the invention may be used alone or in any combination depending on the needs and preferences of the user.