Patent Application
20110232747
This invention relates to thick-film pastes and processes for using such pastes to make solar cell contacts and other circuit devices.
Solar cells are typically made of a semiconductor material, e.g., silicon, which converts sunlight into useful electrical energy. Such solar cells comprise thin wafers of silicon, in which a PN junction is formed by diffusing phosphorus (P) from a suitable phosphorus source into a p-type silicon wafer. The side of the silicon wafer on which sunlight falls is often coated with an anti-reflective coating (ARC) to prevent reflective loss of incoming sunlight, thus increasing the efficiency of the solar cell. A two-dimensional electrode grid pattern, known as a “front contact,” makes a connection to the n-side of the silicon, and a coating of aluminum (Al) on the opposite side (back contact) makes connection to the p-side of the silicon. These contacts are the electrical outlets from the PN junction to the outside load.
The front contacts of silicon solar cells are generally formed by screen-printing a thick-film paste. Typically, the paste contains fine silver particles (75-80 wt %), glass (1-5 wt %), and an organic medium (15-20 wt %). After screen-printing, the wafer and paste are fired in air, typically at furnace set temperatures of about 650-1000° C. for a few seconds to form a dense solid of highly conductive silver traces. During this step, glass reacts with the anti-reflective coating, etches the silicon surface, and facilitates the formation of intimate silicon-silver contact. The organic components are also burned away in this step.
Although silver is a highly electrically conductive metal, it is also expensive and is periodically in short supply. This has motivated attempts to substitute less-expensive metals for at least a portion of the silver in thick-film pastes. Thick-film paste compositions have been disclosed in which 1-90 wt % of the silver has been replaced by nickel or a nickel alloy.
Nevertheless, it is desirable to develop thick-film paste compositions that provide improved performance properties (e.g., efficiency, fill factor, and adhesion) when used in PV devices and other applications.
One aspect of this invention is a thick-film paste comprising:
a) a conductive metal portion comprising a silver component and a nickel component;
b) a frit component comprising lead-tellurium-oxide; and
c) an organic vehicle.
Another aspect of this invention is a process for making a solar cell contact comprising:
a) applying a thick-film paste to a silicon wafer, wherein the thick-film paste comprises:
Another aspect of this invention is a solar cell comprising a front contact, wherein the front contact is formed by firing a thick-film paste of this invention.
In addition to solar cells, the pastes of the invention can be used to produce a variety of circuit devices.
The features of the invention are hereinafter more fully described and particularly pointed out in the claims. The following description sets forth in detail certain illustrative embodiments of the invention, these being indicative of but a few of the various ways in which the invention may be employed.
The thick-film pastes described herein can be used to make front contacts for silicon-based solar cells to collect current generated by exposure to light. The pastes can also be used to make back contacts to conduct electrons to an outside load. The pastes can also be used to create tabs used on solar cells.
Conductive metal portion. The conductive metal portion of the thick-film paste comprises a silver component and a nickel component, typically from about 10-99.9 wt % silver and 0.1-90 wt % nickel; or about 50-99.9 wt % silver and 0.1-50 wt % nickel; or about 70-99.9 wt % silver and 0.1-30 wt % nickel; or about 80-99.9 wt % silver and 0.1-20 wt % nickel, wherein the silver (or nickel) weight percent is calculated on the basis of the silver (or nickel) content of the conductive metal portion.
Typically, the conductive metal portion comprises 50-95 wt % of the thick-film paste, and is calculated on the basis of the silver components (e.g., silver metal particles or silver salts) and nickel components (e.g., nickel metal or nickel alloy).
Silver Component. The silver can be in the form of silver metal, alloys of silver, or mixtures thereof. The silver can also be in the form of silver oxide (Ag2O), silver salts such as AgCl, AgNO3, AgOOCCH3 (silver acetate), AgOOCF3 (silver trifluoroacetate), silver orthophosphate (Ag3PO4), or mixtures thereof. Other forms of silver compatible with the other thick-film paste components can also be used. Particles of silver or silver alloy can be coated with other materials, e.g., phosphorus. Alternatively, silver can be coated onto glass, or silver oxide can be dissolved in the glass during the glass melting/manufacturing process.
In one embodiment, the solids portion of the thick-film paste composition comprises about 80 to about 90 wt % silver particles and about 1 to about 9.5 wt % silver flakes.
In one embodiment, the thick-film paste composition comprises coated silver particles that are electrically conductive. Suitable coatings include phosphorous and surfactants. Suitable surfactants include polyethyleneoxide, polyethyleneglycol, benzotriazole, poly(ethyleneglycol)acetic acid, lauric acid, oleic acid, capric acid, myristic acid, linolic acid, stearic acid, palmitic acid, stearate salts, palmitate salts, and mixtures thereof. The salt counter-ions can be ammonium, sodium, potassium or mixtures thereof.
The particle size of the silver is not subject to any particular limitation. In one embodiment, an average particle size is less than 10 microns; in another embodiment, the average particle size is less than 5 microns; in another embodiment, the average particle size is less than 3 microns; in another embodiment, the average particle size is less than 1 micron; in another embodiment, a mixture of particle sizes is used.
Nickel Component. The nickel component is selected from the group consisting of nickel metal and nickel alloys. Nickel metal is typically in the form of powders or flakes. In some embodiments, the nickel particles have an average particle size of between 0.2 and 10.0 microns, e.g., about 0.5 microns, 1.0 micron, 2.5 microns, 5.0 microns, 6.6 microns, or 10 microns. In some embodiments, the nickel particle size is approximately the same as the silver particle size.
The nickel can be provided in the form of essentially pure nickel powder, nickel flake, colloidal nickel, or as nickel alloyed with one or more other metals including any or all of aluminum, chromium, silicon, iron, molybdenum, antimony, vanadium and niobium/tantalum. As is known in the art, the proportion of niobium and tantalum are given together owing to their tendency to intimately alloy/mix and the difficulty in purifying one from the other.
In some embodiments, the thick-film paste comprises a nickel alloy comprising about 70-99 wt % nickel and about 1-30 wt % aluminum, or about 80-99 wt % nickel and 1-20 wt % aluminum, or about 80-96 wt % nickel and about 4-20 wt % aluminum.
Instead of, or in addition to, a Ni—Al alloy, a Ni—Cr alloy may be present in the pastes, such as Ni—Cr alloy comprising about 48-81 wt % nickel and about 19-52 wt % chromium. In another embodiment of the invention, the paste comprises a Ni—Cr alloy, the Ni—Cr alloy comprising about 1-60 wt % chromium.
In another embodiment, the conductive metal portion comprises from about 10-99.9 wt % silver, and (b) from about 0.1-90 wt % of a nickel alloy selected from the group consisting of a Ni—Al alloy, a Ni—Cr alloy, a Ni—Al—Cr alloy, and combinations thereof.
When a Ni—Al alloy is used, it may comprise about 1-30 wt % aluminum and about 70-99 wt % nickel. In yet another embodiment, the conductive metal portion comprises (a) about 37.5-75 wt % silver and (b) about 25-62.5 wt % of a nickel alloy. In still another embodiment, the conductive metal portion comprises (a) about 13.8-87.5 wt % silver and (b) about 12.5-86.2 wt % of a nickel alloy selected from the group consisting of Ni—Al, Ni—Cr, Ni—Al—Cr, and combinations thereof.
The nickel alloy can further comprise an element selected from the group consisting of cobalt, iron, silicon, molybdenum, niobium, tantalum, manganese, vanadium, antimony, boron, and combinations thereof. For example, some embodiments can include at least one of the following: about 1-30 wt %, or about 5-25 wt %, or about 10-20 wt % chromium; about 0.1-10 wt %, or about 0.3-8 wt %, or about 1-5 wt % iron; about 0.1-5 wt %, or about 1-4 wt %, or about 1.5-3 wt % silicon; about 1-10 wt %, or about 2-8 wt %, or about 3-7 wt % molybdenum; about 0.1-5 wt %, or about 0.25-4 wt % manganese; about 0.1-10 wt %, or about 0.3-8 wt %, or about 1-5 wt % niobium+tantalum; about 0.5-8 wt %, or about 1-7 wt %, or about 2-6 wt % vanadium; and 0.5-9 wt %, or about 1-8 wt %, or about 2-6 wt % antimony.
The metals and alloys can be provided in the form of powders, flakes, or colloids. The particles of the metals silver, nickel, aluminum, (Al alone being only in a back contact) and alloys containing combinations of silver, aluminum, nickel and nickel alloys, have an average size of less than about 10 microns, or less than about 5 microns, or less than about 1 micron. In other embodiments, the metal or alloy particles have average particle sizes of less than about 750 nm, less than about 500 nm or less than about 250 nm.
Frit Component. The frit component comprises a lead-tellurium oxide and optionally other metal compounds, metal oxides or glasses. Typically, the frit comprises 1-10 wt % of the thick-film paste, based on the weight of the solids.
The lead-tellurium-oxide (Pb—Te—O) can be prepared by mixing TeO2 and PbO powders, heating the powder mixture in air or an oxygen-containing atmosphere to form a melt, quenching the melt, grinding and ball-milling the quenched material, and screening the milled material to provide a powder with the desired particle size. Firing the mixture of lead and tellurium oxides is typically conducted to a peak temperature of 800 to 1200° C. The molten mixture can be quenched, for example, on a stainless steel platen or between counter-rotating stainless steel rollers to form a thick platelet. The resulting platelet can be milled to form a powder. Typically, the milled powder has a D50 of 0.1-3.0 microns.
Typically, the mixture of PbO and TeO2 powders comprises 5-95 mol % of lead oxide and 5-95 mol % of tellurium oxide, based on the combined powders. In one embodiment, the mixture of PbO and TeO2 powders comprises 30-85 mol % of lead oxide and 15-70 mol % of tellurium oxide, based on the combined powders.
In some embodiments, the mixture of PbO and TeO2 powders further comprises one or more other metal compounds. Suitable other metal compounds include PbF2, SiO2, B2O3, Li2O, Na2O, K2O, Rb2O, Cs2O, MgO, CaO, SrO, BaO, TiO2, V2O5, ZrO, MoO3, Mn2O3, Ag2O, ZnO, Ga2O3, GeO2, In2O3, SnO2, Sb2O3, Bi2O3, P2O5, CuO, SeO2, and CeO2.
Table 1 lists some examples of powder mixtures containing PbO, TeO2 and other optional metal compounds that can be used to make lead-tellurium oxides. This list is meant to be illustrative, not limiting.
As used herein, the term “Pb—Te—O” refer to compositions that comprise lead-tellurium oxides and may further comprise metal oxides or carbonates that contain one or more elements selected from the group consisting of Si, Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, V, Zr, Mo, Mn, Zn, B, P, Sn, Ga, Ge, In, Sb, Bi, Ce, Cu, and Ag.
In some embodiments, the lead-tellurium-oxide frit component comprises a mixture of a lead-tellurium-oxide and a tellurium-free oxide, e.g., a lead-lithium-boron-silion-aluminum-zirconium-sodium-oxide.
Organic Vehicle. The organic vehicle is typically a solution of a resin dissolved in a solvent, optionally further comprising a thixotropic agent. The solvent typically has a boiling point between about 130 and 350° C. In some embodiments, the resin is ethyl cellulose, ethyl hydroxyethyl cellulose, wood rosin, a mixture of ethyl cellulose and phenolic resins, a polymethacrylate, the monobutyl ether of ethylene glycol monoacetate, or mixtures thereof.
Suitable solvents include terpenes (e.g., alpha- or beta-terpineol; Hercules Inc., Wilmington, Del.); Dowanol® (diethylene glycol monoethyl ether; Dow Chemical Co., Midland, Mich.); butyl Carbitol® (diethylene glycol monobutyl ether; Dow Chemical Co.); dibutyl Carbitol® (diethylene glycol dibutyl ether; Dow Chemical Co.); butyl Carbitol® acetate (diethylene glycol monobutyl ether acetate; Dow Chemical Co.); hexylene glycol; Texanol® (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate; Eastman Chemical Company, Kingsport, Tenn.); alcohol esters; kerosene; dibutyl phthalate and mixtures thereof. Other suitable organic medium components include: bis(2-(2-butoxyethoxy)ethyl adipate, dibasic esters such as DBE, DBE-2, DBE-3, DBE-4, DBE-5, DBE-6, DBE-9, and DBE 1B, octyl epoxy tallate, isotetradecanol, and pentaerythritol ester of hydrogenated rosin. The organic medium can also include volatile liquids to promote rapid hardening after application of the thick-film paste composition on a substrate.
Various combinations of these and other solvents can be formulated to obtain the desired viscosity and volatility requirements for each application.
Suitable organic thixotropic agents include hydrogenated castor oil and derivatives thereof. A thixotrope is not always necessary because the solvent/resin properties, coupled with the shear thinning inherent in any suspension, may be sufficient. Suitable wetting agents include fatty acid esters and diamines, e.g., N-tallow-1,3-diaminopropane di-oleate, N-tallow trimethylene diamine diacetate, N-coco trimethylene diamine, beta-diamines, N-oleyl trimethylene diamine, N-tallow trimethylene diamine, N-tallow trimethylene diamine dioleate, and combinations thereof.
The organic vehicle can further comprise dispersants, surfactants and/or rheology modifiers, which are commonly used in thick-film paste formulations.
The optimal amount of organic medium in the thick-film paste composition is dependent on the method of applying the paste and the specific organic medium used. Typically, the thick-film paste composition contains 70 to 95 wt % of inorganic components and 5 to 30 wt % of organic medium.
If the organic medium comprises a polymer, the polymer may constitute 8 to 15 wt % of the organic medium.
Inorganic/Other Additives. Phosphorus can be added to the paste in a variety of ways to reduce the resistance of the front contacts. For example, certain glasses can be modified with P2O5 in the form of a powdered or fritted oxide, or phosphorus can be added to the paste by way of phosphate esters and other organo-phosphorus compounds. Phosphorus can also be added as a coating to silver and/or nickel and or Ag/Ni alloy particles prior to making a paste. In such case, the silver and/or nickel and or Ag/Ni alloy particles are mixed with liquid phosphorus and a solvent. For example, a blend of about 85-95 wt % silver and/or nickel and/or Ag/Ni particles, about 5-15 wt % solvent and about 0.5-10 wt % liquid phosphorus are mixed and the solvent evaporated. Phosphorus-coated silver and/or nickel and/or Ag/Ni alloy particles help ensure intimate mixing of phosphorus and silver and/or nickel and/or Ag/Ni alloy in the pastes.
Other additives such as fine silicon and/or carbon powder can be added to the paste to control the silver reduction and precipitation reaction. The silver precipitation at the interface or in the bulk glass can also be controlled by adjusting the firing atmosphere (e.g., by firing in flowing N2 or N2/H2/H2O mixtures). However, no special atmosphere is required. Fine low-melting metal additives (i.e., elemental metallic additives as distinct from metal oxides) such as Pb, Bi, In, Ga, Sn, Ni, and Zn or alloys of each with at least one other metal can be added to provide a contact at a lower firing temperature, or to widen the firing window. Typically such metal additives are present at less than about 1 wt % of the conductive metal portion of the pastes. Organometallic compounds providing aluminum, barium, bismuth, magnesium, zinc, strontium and potassium can be used, such as, for example, the acetates, acrylates, formates, neodeconates, methoxides, ethoxides, methoxyethoxides, and stearates of the named metals. Potassium silicate is also a suitable source of potassium.
Embodiments. In one embodiment, the thick-film paste comprises an organic vehicle, a lead-tellurium-oxide frit component and a conductive metal portion, wherein the conductive metal portion comprises: (a) from about 10-99 wt % silver and (b) from about 1-90 wt % of a nickel alloy selected from the group consisting of a Ni—Al alloy, a Ni—Cr alloy, a Ni—Al—Cr alloy, and combinations thereof.
Another embodiment of the invention is a thick-film paste comprising: (a) a lead-tellurium-oxide frit component comprising frit particles having a particle size no greater than about 2 microns, and (b) a conductive metal portion comprising: (i) from about from about 10-99 wt % silver and (ii) from about 0.05-90 wt % of a nickel alloy selected from the group consisting of a Ni—Al alloy, a Ni—Cr alloy, a Ni—Al—Cr alloy, and combinations thereof.
Another embodiment of the invention is a thick-film paste comprising: (a) a lead-tellurium-oxide frit component; and (b) a metal portion comprising about 5-85 wt % nickel; about 10-80 wt % silver, and about 0.1-10 wt % of a metal selected from the group consisting of aluminum, chromium, and combinations thereof.
Another embodiment of the invention is a thick-film paste comprising: a lead-tellurium-oxide frit component and a conductive metal portion, said conductive metal portion comprising: from about 10-99 wt % silver and from about 1-90 wt % of a nickel alloy selected from the group consisting of a Ni—Al alloy, a Ni—Cr alloy, a Ni—Al—Cr alloy, and combinations thereof.
Another embodiment of the invention is a thick-film paste comprising a vehicle, a lead-tellurium-oxide frit component and a conductive metal portion, where the conductive metal portion comprises: (a) from about 10-99 wt % silver and (b) from about 1-90 wt % of a nickel alloy selected from the group consisting of a Ni—Al alloy, a Ni—Cr alloy, a Ni—Al—Cr alloy, and combinations thereof.
Another embodiment of the invention is a thick-film paste comprising a metal portion comprising about 5-85 wt % nickel; about 10-80 wt % silver and about 0.1-10 wt % of a metal selected from the group consisting of aluminum, chromium, and combinations thereof.
Another embodiment of the invention is a thick-film paste comprising (a) a lead-tellurium-oxide frit component comprising frit particles having a particle size no greater than about 2 microns, and (b) a conductive metal portion comprising (i) from about from about 10-99 wt % silver and (ii) from about 0.05-90 wt % of a nickel alloy selected from the group consisting of a Ni—Al alloy, a Ni—Cr alloy, a Ni—Al—Cr alloy, and combinations thereof.
If a nickel alloy is used, the thick-film paste may comprise as little as about 10% by weight silver. Use of the nickel in the paste provides a distinct advantage, since the cost of nickel is typically less than half of the cost of silver.
Solar cells. Another embodiment of the invention is a solar cell comprising an electrical contact, wherein the contact comprises a metal portion comprising, prior to firing: about 5-85 wt % nickel; about 10-80 wt % silver, and about 0.1-10 wt % of a metal selected from the group consisting of aluminum, chromium, and combinations thereof.
Another embodiment of the invention is a solar cell comprising a front electrical contact, said front electrical contact formed by firing a thick-film paste composition comprising a lead-tellurium-oxide frit component and a conductive metal portion, said conductive metal portion comprising silver and at least about 8 wt % nickel.
Another embodiment of the invention relates to a process for making a solar cell contact, the process comprising (a) applying a thick-film paste to a silicon wafer, wherein the thick-film paste comprises (i) a lead-tellurium-oxide frit component and (ii) a conductive metal portion, said conductive metal portion comprising (1) from about 10-99.1 wt % silver and (2) from about 0.1-90 wt % of a nickel alloy selected from the group consisting of a Ni—Al alloy, a Ni—Cr alloy, a Ni—Al—Cr alloy, and combinations thereof, and (b) firing the silicon wafer at a time and temperature sufficient to sinter the metal portion.
Another embodiment of the invention is a process for making a solar cell contact, comprising (a) applying a thick-film paste to a silicon wafer, wherein the thick-film paste comprises (i) a lead-tellurium-oxide frit component and (ii) a conductive metal portion, said conductive metal portion comprising (1) silver and (2) at least about 0.1 wt % nickel, and (b) firing the silicon wafer at a time and temperature sufficient to sinter the metal portion.
Another embodiment of the invention is a solar cell comprising a front electrical contact, where the front electrical contact is formed by firing a thick-film paste composition comprising a lead-tellurium-oxide frit component and a conductive metal portion. The conductive metal portion comprises Ag and at least about 1 wt % nickel.
Another embodiment of the invention is a solar cell comprising an electrical contact, wherein the electrical contact comprises a metal portion comprising, prior to firing: about 5-85 wt % nickel; about 10-80 wt % silver, and about 0.1-10 wt % of a metal selected from the group consisting of aluminum, chromium, and combinations thereof.
Another embodiment is a solar cell front electrical contact comprising silver, a nickel alloy, and aluminum.
Another embodiment of the invention is a process for making a solar cell electrical contact, the process comprising (a) applying a thick-film paste to a silicon wafer, wherein the thick-film paste comprises (i) a lead-tellurium-oxide frit component and (ii) a conductive metal portion, said conductive metal portion comprising (1) from about 10-99.9 wt % silver and (2) from about 0.1-90 wt % of a nickel alloy selected from the group consisting of a Ni—Al alloy, a Ni—Cr alloy, a Ni—Al—Cr alloy, and combinations thereof, and (b) firing the silicon wafer at a time and temperature sufficient to sinter the metal portion.
Another embodiment of the invention is a process for making a solar cell contact, comprising (a) applying a thick-film paste to a silicon wafer, wherein the paste comprises (i) a lead-tellurium-oxide frit component and (ii) a conductive metal portion, said conductive metal portion comprising (1) silver and (2) at least about 1 wt % nickel, and (b) firing the silicon wafer at a time and temperature sufficient to sinter the metal portion.
Preparation of the Thick-Film Paste Composition. A Paste According to the invention can be prepared either by mixing the individually prepared silver and nickel paste in various proportions or by mixing silver and nickel powder in required proportions prior to making the paste.
In one embodiment, the thick-film paste composition can be prepared by mixing the conductive metal powder, the lead-tellurium-oxide powder, and the organic medium in any order. In some embodiments, the inorganic materials are mixed first, and they are then added to the organic medium. The viscosity can be adjusted, if needed, by the addition of solvents. Mixing methods that provide high shear may be useful.
Another aspect of the present invention is a process comprising:
a) providing an article comprising one or more insulating films disposed onto at least one surface of a semiconductor substrate;
(b) applying a thick-film paste composition onto at least a portion of the one or more insulating films to form a layered structure, wherein the thick-film paste composition comprises:
In one embodiment, a semiconductor device is manufactured from an article comprising a junction-bearing semiconductor substrate and a silicon nitride insulating film formed on a main surface thereof. The process includes the steps of applying (e.g., coating or screen-printing) onto the insulating film, in a predetermined shape and thickness and at a predetermined position, a thick-film paste composition having the ability to penetrate the insulating layer, then firing so that thick-film paste composition reacts with the insulating film and penetrates the insulating film, thereby effecting electrical contact with the silicon substrate.
One embodiment of this process is illustrated in
In
After protecting one surface of the n-type diffusion layer 20 with a resist or the like, as shown in
Next, in
Next, electrodes are formed. As shown in
Consequently, as shown in
On the front-side, the thick-film paste composition 500 of the present invention sinters and penetrates through the insulating film 30 during firing, and thereby achieves electrical contact with the n-type diffusion layer 20. This type of process is generally called “fire through.” This fired-through state, i.e., the extent to which the paste melts and passes through the insulating film 30, depends on the quality and thickness of the insulating film 30, the composition of the paste, and on the firing conditions. When fired, the paste 500 becomes the electrode 501, as shown in
In one embodiment, the insulating film is selected from titanium oxide, aluminum oxide, silicon nitride, SiNx:H, silicon oxide, and silicon oxide/titanium oxide films. The silicon nitride film can be formed by sputtering, a plasma enhanced chemical vapor deposition (PECVD), or a thermal CVD process. In one embodiment, the silicon oxide film is formed by thermal oxidation, sputtering, or thermal CFD or plasma CFD. The titanium oxide film can be formed by coating a titanium-containing organic liquid material onto the semiconductor substrate and firing, or by a thermal CVD.
In this process, the semiconductor substrate can be a single-crystal or multi-crystalline silicon electrode.
Suitable insulating films include one or more components selected from: aluminum oxide, titanium oxide, silicon nitride, SiNx:H, silicon oxide, and silicon oxide/titanium oxide. In one embodiment of the invention, the insulating film is an anti-reflection coating (ARC). The insulating film can be applied to a semiconductor substrate, or it can be naturally forming, such as in the case of silicon oxide.
In one embodiment, the insulating film comprises a layer of silicon nitride. The silicon nitride can be deposited by CVD (chemical vapor deposition), PECVD (plasma-enhanced chemical vapor deposition), sputtering, or other methods.
In one embodiment, the silicon nitride of the insulating layer is treated to remove at least a portion of the silicon nitride. The treatment can be a chemical treatment. The removal of at least a portion of the silicon nitride may result in an improved electrical contact between the conductor of the thick-film paste composition and the semiconductor substrate. This may result in improved efficiency of the semiconductor device.
In one embodiment, the silicon nitride of the insulating film is part of an anti-reflective coating.
The thick-film paste composition can be printed on the insulating film in a pattern, e.g., bus bars with connecting lines. The printing can be by screen printing, plating, extrusion, inkjet, shaped or multiple printing, or ribbons.
In this electrode-forming process, the thick-film paste composition is heated to remove the organic medium and sinter the metal powder. The heating can be carried out in air or an oxygen-containing atmosphere. This step is commonly referred to as “firing.” The firing temperature profile is typically set so as to enable the burnout of organic binder materials from dried thick-film paste composition, as well as any other organic materials present. In one embodiment, the firing temperature is 750 to 950° C. The firing can be conducted in a belt furnace using high transport rates, for example, 100-500 cm/min, with resulting hold-up times of 0.05 to 5 minutes. Multiple temperature zones, for example 3-11 zones, can be used to control the desired thermal profile.
Upon firing, the electrically conductive metal and lead-tellurium-oxide frit component mixture penetrate the insulating film. The penetration of the insulating film results in an electrical contact between the electrode and the semiconductor substrate. After firing, an interlayer may be formed between the semiconductor substrate and the electrode, wherein the interlayer comprises one or more of tellurium, tellurium compounds, lead, lead compounds, and silicon compounds, where the silicon may originate from the silicon substrate and/or the insulating layer(s). After firing, the electrode comprises sintered metal that contacts the underlying semiconductor substrate and may also contact one or more insulating layers.
Another aspect of the present invention is an article formed by a process comprising:
(a) providing an article comprising one or more insulating films disposed onto at least one surface of a semiconductor substrate;
(b) applying a thick-film paste composition onto at least a portion of the one or more insulating films to form a layered structure,
wherein the thick-film paste composition comprises:
Such articles may be useful in the manufacture of photovoltaic devices. In one embodiment, the article is a semiconductor device comprising an electrode formed from the thick-film paste composition. In one embodiment, the electrode is a front-side electrode on a silicon solar cell. In one embodiment, the article further comprises a back electrode. It will be appreciated that although the examples herein primarily concern a conductive composition for use in forming a conductor paste for use in the formation of solar cell contacts, the present invention also contemplates the use of the principles disclosed herein to form resistor pastes, semiconductor pastes, inks, or tapes. Furthermore, such compositions may be considered as materials for use in forming thick films. Thus, the compositions disclosed herein can be used to form conductive, resistive or semiconducting paths or patterns on substrates. Such conductive compositions can assume various forms, including an ink, a paste, or a tape. Deposition of the composition on a substrate can be by screen printing, plating, extrusion, inkjet, contact printing, stencil printing, shaped or multiple printing, or ribbons.
Substrates other than silicon may be employed in connection with the pastes of the present invention. The use of the compositions disclosed herein in a variety of electronic components and devices is also envisioned.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and illustrative examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general invention concept as defined by the appended claims and their equivalents.
Thick-film pastes were prepared by mixing the individually prepared silver and nickel pastes in various proportions. The silver and nickel pastes were made as detailed below.
Frit Preparation
Mixtures of TeO2 powder (99+% purity), PbO powder (ACS reagent grade, 99+% purity) and Li2CO3 in the % cation ratio Te:Pb:Li of 57:38:5 were tumbled in a polyethylene container for 30 min to mix the starting powders. The starting powder mixture was placed in a platinum crucible and heated in air at a heating rate of 10° C./min to 900° C. and then held at 900° C. for one h to melt the mixture. The melt was quenched from 900° C. by removing the platinum crucible from the furnace and pouring the melt onto a stainless steel platen. The resulting material was ground in a mortar and pestle to less than 100 mesh. The ground material was then ball-milled in a polyethylene container with zirconia balls and isopropyl alcohol until the D50 was 0.5-0.7 microns. The ball-milled material was then separated from the milling balls, dried, and run through a 100 mesh screen to provide the PbO—TeO2—Li2O powder (PTOL) used in the thick-film paste preparations.
Thick-Film Paste Preparation
Silver Paste: A 50 g batch of silver paste was made by placing silver powder (44.72 g) in a glass jar, to which was added 1.04 g of PbO—TeO2—LiO2 powder (PTOL), prepared as above. The powders were then tumble-mixed for about 15 min. An organic medium containing solvents and binders was prepared by mixing the various components in respective amounts as listed in Table 2 in a plastic jar using a planetary centrifugal mixer, THINKY® ARE-310 (THINKY USA, Inc., Laguna Hills, Calif.) for 1 min at 2000 rpm. To this organic medium, approximately one third of the silver and PTOL powder mix was added and mixed using THINKY® ARE-310 for 1 min at 2000 rpm. This step was repeated with the second and third portion of silver and PTOL powder mix, making sure the inorganic powder mix was thoroughly dispersed in the medium. The dispersed mixture was then blended with triple roll mill (Charles Ross & Son Company, Floor Model, 4″×8″) at a 1 mil gap for three passes at zero psi and three passes at 100 psi to obtain a thick paste. The viscosity of the blended paste was adjusted with 0.14 g of Texanol (Eastman Chemical Company, TN) to obtain a printable paste.
The solid content of final paste was measured in duplicate by weighing small quantities (1-2 g) into an alumina boat and firing in a muffle furnace at 450° C. for 30 min to remove organics, and reweighing the alumina boat and contents. The average solid content of the paste was determined to be 90.2%.
The paste viscosity was measured using a Brookfield HADV-I Prime viscometer (Brookfield Engineering Laboratories, Inc., Middleboro, Mass.) with the thermostatted small-sample adapter at about 10 rpm and was found to be 278 Pas.
Nickel paste: A 50 g batch of nickel paste was made following the procedure described above for the preparation of the silver paste, except that nickel powder was used in place of silver powder. The solid content of final paste was measured to be 90.8% and its viscosity was measured to be 170 Pas.
Silver/Nickel Pastes: Paste containing silver and nickel in approximately a 3:1 ratio (i.e., approximately 75 wt % silver and 25 wt % nickel) was prepared by mixing silver paste and nickel paste in a 3:1 weight ratio in a THINKY® ARE-310 planetary mixture for 1 min at 2000 rpm. Similarly, a paste containing approximately 50 wt % silver and 50 wt % nickel was prepared by mixing silver paste and nickel paste in a 1:1 weight ratio. For each paste mixture, the mixing was repeated for three more times to obtain a thoroughly mixed blended paste. Pastes formulations prepared using PTOL frit are labeled as “Paste A”
Solar cells for testing the performance of the thick-film pastes were made from 200 micron-thick multi-crystalline silicon wafers (Deutsche Solar AG) with a 65 ohm/sq phosphorus-doped emitter layer which had an acid-etched textured surface and 70-80 nm thick PECVD SiNx antireflective coating. The wafers were cut into 28 mm×28 mm wafers using a diamond wafering saw.
Wafers were screen-printed full ground-plane with commercial aluminum paste, PV381 (E. I. du Pont de Nemours and Company, Wilmington, Del.) for back side contact using a screen on an 8″×10″ frame (Sefar Inc., Depew, N.Y.) with a square opening of 26.99 mm×26.99 mm and a screen printer, MSP 885 (Affiliated Manufacturers Inc., North Branch, N.J.). This left a nominal 0.5 mm border of bare Si (i.e., without aluminum paste) around the edges. The wet weight of aluminum paste was targeted to be around 60 mg, which produced an aluminum loading after firing of about 5.9 mg Al/cm2. After printing, the aluminum paste was dried in a mechanical convection oven with vented exhaust for 30 min at 150° C., resulting in a dried film thickness of 25-30 microns.
The silver paste, or nickel blend silver pastes, were screen-printed using a screen on 8″×10″ frames (Sefar Inc.) and a screen printer, MSP 485 (Affiliated Manufacturers Inc.) on the silicon nitride layer of the front surface of silicon wafers and dried at 150° C. for 30 min in a convection oven to give 25-30 microns thick grid lines and a bus bar. The screen-printed silver paste (or silver-nickel paste) had a pattern of eleven fingers/grid lines of 100-125 microns width, connected to a bus bar of 1.25 mm width located near one edge of the cell. The screen for printing the silver paste used 325-mesh wires of 23 microns diameter at 30° angle and 32 microns thick emulsion.
The dried cells were fired in a 4-zone furnace (BTU International, North Billerica, Mass.; Model PV309) at a belt speed of 221 cm/min, with the following furnace setpoint temperatures: zone 1 at 610° C., zone 2 at 610° C., zone 3 at 585° C., and the final zone 4 set at peak temperature, Tmax, in the range of 860° C. to 960° C. The wafers took about 5.2 sec to pass through zone 4. In Table 3, only the peak firing temperature of zone 4 is reported, which is approximately 100-125° C. greater than the actual wafer temperature. After the firing process, the wafer is a functional photovoltaic cell. For each paste composition, a number of duplicate photovoltaic cells were fabricated. These photovoltaic cells were divided into 4-5 groups, with 4-5 cells in each group, all fired at the same temperature. Each cell group gave optimum median cell efficiency at a firing temperature, which might be different for the different paste compositions.
A commercial Current-Voltage (JV) tester ST-1000 (Telecom-STV Ltd., Moscow, Russia) was used to make efficiency measurements of the polycrystalline silicon photovoltaic cells. Two electrical connections, one for voltage and one for current, were made on the top and the bottom of each of the photovoltaic cells. Transient photo-excitation was used to avoid heating the silicon photovoltaic cells and to obtain JV curves under standard temperature conditions (25° C.). A flash lamp with a spectral output similar to the solar spectrum illuminated the photovoltaic cells from a vertical distance of 1 m. The lamp power was held constant for 14 milliseconds. The intensity at the sample surface, as calibrated against external solar cells was 1000 W/m2 (or 1 Sun) during this time period. During the 14 milliseconds, the JV tester varied an artificial electrical load on the sample from short circuit to open circuit. The JV tester recorded the light-induced current through, and the voltage across, the photovoltaic cells while the load changed over the stated range of loads. A power versus voltage curve was obtained from this data by taking the product of the current times the voltage at each voltage level. The maximum of the power versus voltage curve was taken as the characteristic output power of the solar cell for calculating solar cell efficiency. This maximum power was divided by the area of the sample to obtain the maximum power density at 1 Sun intensity. This was then divided by 1000 W/m2 of the input intensity to obtain the efficiency which is then multiplied by 100 to present the result in percent efficiency. Other parameters of interest were also obtained from this same current-voltage curve. One such parameter is fill factor (FF) which is obtained by taking the ratio of the maximum power from the solar cell to the product of open circuit voltage and short circuit current. For reasonably efficient cells, an estimate of the series resistance (Rseries) was obtained from the reciprocal of the local slope of the current voltage curve near the short circuit point. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc.
Median values for optimum cell efficiency, fill factor and series resistance, for solar cells prepared using the thick-film pastes of Examples 1-3 are summarized in Table 3.
These results demonstrate that the Ag/Ni blend thick-film pastes can provide solar cells with good performance characteristics.
Paste adhesion to the silicon wafer is a critical performance requirement for stability and long-term durability of solar cell devices. As described below, adhesion can be assessed by attaching the fired paste to a solder ribbon, and then pulling the soldered tab and measuring the force required at break. Fired pastes exceeding 2.5 N force at break are generally considered to meet the industry requirement.
Test samples for the adhesion measurements were printed and fired the same way as detailed in ‘solar cell fabrication’ section, except that the samples were printed with three bus bars instead of grid lines and a bus bar using a screen on 8″×10″ frame (Sefar Inc.) with 325-mesh wires of 27 microns diameter at 30° angle and 27 microns thick emulsion.
The print bar of the fired test sample was 2 mm×20 mm, with 4 mm spacing between the bars. The back side of the fired test sample was glued onto an alumina substrate using a 2-part epoxy, Hardman® (Royal adhesives and sealants, CA), and cured for at least for 15 min. A 2 mm wide, 3″ long, tinned-copper ribbon (Sn/Cu/Ag in 62/36/2 ratio; Ulbrich Inc, CT) was cut and flattened, and then a thin layer of no-clean flux, 959T (Kester Inc, IL) was applied onto a 1″ long portion and dried for 15 min. The dried flux-coated portion was then placed on top of the fired bus bar. Heat at 320° C. was applied with the solder rod for approximately 5 sec, resulting in the attachment of the ribbon to the fired paste. This step was repeated for the other two bus bars of the sample.
The adhesion test was performed by pulling the soldered tab at a 90° angle and measuring the force required at break with Instron® Model 5569 (Instron Inc, MA). The average force required to pull each tab was recorded. Four samples for each paste were tested, for a total of 12 tab pulls per paste composition. The average data from 4 samples are presented in Table 4.
The thick-film paste used for Comparative Example B is similar to that of Example 1, but it does not contain PTOL.
These results demonstrate that a paste of this invention containing Ag, Ni and PTOL has a significant improvement in adhesion to silicon wafers, relative to Comparative Examples A-C, which lack either Ni or PTOL.
Paste preparations were accomplished with the following procedure: The appropriate amount of solvent, medium and surfactant was weighed then mixed in a Thinky® mixer for 30-60 sec, then glass frits and metal additives were added and mixed for 1-2 min. When well-mixed, the paste was passed through a 3-roll mill a few times, at 0-250 psi. The gap of the rolls were typically adjusted to 1 mil. The degree of dispersion was measured by fineness of grind (FOG). A typical FOG value is generally equal to or less than 20/15 for conductors.
Compositions of the nickel, nickel alloy powders, lead-tellurium oxides, and organic media used in Examples 4-21 are described in Tables 5-7. Table 8 shows the metal (Ag+Ni or Ni alloy) and lead-tellurium-oxide (glass frit) compositions in wt % of the total composition.
The solar cells built according to the method described above were placed in a Berger IV tester to measure the efficiencies. The light bulb in the IV tester simulated the sunlight with a known intensity and irradiated the front surface of the cell. The bus bars printed in the front of the cell were connected to the multiple probes of the IV tester and the electrical signals were transmitted through the probes to the computer for calculating efficiencies.
After firing, a solder ribbon (copper coated with 62Sn/36Pb/2Ag) was soldered to the bus bars printed on the front of the cell. The soldering was typically carried out at 200° C. for 1-2 sec. The flux used was Kester® 959. The soldered area was approximately 1.8 mm×145 mm. The adhesion strength was obtained by pulling the ribbon at an angle of 180° to the surface of the cell at a speed of 120 mm/min.
In similar experiments using Ni powders of different sizes and 6 inch wafers, the following results were obtained, as shown in Tables 10 and 11. In these examples, the frit component comprises Glass II and Glass IV (see Table 8).
This application claims the benefit of U.S. Provisional Patent Application No. 61/331,006 filed May 4, 2010, U.S. Provisional Patent Application No. 61/440,117 filed Feb. 7, 2011, U.S. Provisional Patent Application No. 61/445,508 filed Feb. 22, 2011, and U.S. Provisional Patent Application No. 61/467,003 filed Mar. 24, 2011, all of which are herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61331006 | May 2010 | US | |
| 61440117 | Feb 2011 | US | |
| 61445508 | Feb 2011 | US | |
| 61467003 | Mar 2011 | US |
