HK1180671B - Thick-film pastes containing lead-tellurium-lithium- oxides, and their use in the manufacture of semiconductor devices - Google Patents

Description

Thick film pastes containing lead-tellurium-lithium-oxides and their use in the manufacture of semiconductor devices

Technical Field

The present invention provides a thick film paste for printing the front side of a solar cell device having one or more insulating layers. The thick film paste comprises a source of a conductive metal or derivative thereof, and a lead-tellurium-lithium-oxide dispersed in an organic medium.

Background

Conventional solar cell structures with p-type substrates have a negative electrode, typically on the front side (the illuminated side) of the cell, and a positive electrode on the back side. Radiation of a suitable wavelength incident on the p-n junction of the semiconductor serves as an external energy source for the charge carriers of the electron-hole pairs. These electron-hole pair charge carriers migrate in the electric field generated by the p-n semiconductor junction and are collected by a conductive grid or metal contact applied to the semiconductor surface. The generated current flows to an external circuit.

Conductive pastes (also known as inks) are commonly used to form conductive grids or metal contacts. The conductive paste generally comprises a glass frit, a conductive substance (e.g., silver particles), and an organic medium. To form the metal contacts, a conductive paste is printed onto the substrate in a grid line or other pattern and then fired, during which electrical contact is made between the grid lines and the semiconductor substrate.

However, crystalline silicon PV cells are typically coated with an antireflective coating (e.g., silicon nitride, titanium oxide, or silicon oxide) to promote light absorption, thereby increasing the efficiency of the cell. Such antireflective coatings also act as insulators, which impair the flow of electrons from the substrate to the metal contacts. To overcome this problem, the conductive paste should penetrate the anti-reflective coating layer during firing to form a metal contact having electrical contact with the semiconductor substrate. It is also desirable to form a strong bond (i.e., adhesion) between the metal contact and the substrate as well as solderability.

The ability to penetrate the antireflective coating and form a strong bond to the substrate upon firing is highly dependent on the composition of the conductive paste and the firing conditions. Efficiency, an important measure of PV cell performance, is also affected by the quality of the electrical contact formed between the fired conductive paste and the substrate.

In order to provide an economical method of manufacturing PV cells with good efficiency, there is a need for thick film paste compositions that can penetrate the antireflective coating and provide good electrical contact with the semiconductor substrate by firing at low temperatures.

Disclosure of Invention

One aspect of the invention is a thick film paste composition comprising:

a) 85-99.75 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

b) 0.25-15 wt% lead-tellurium-lithium-oxide, based on solids; and

c) an organic medium.

Another aspect of the invention is a method comprising:

(a) providing a semiconductor substrate comprising one or more insulating films deposited onto at least one surface of the semiconductor substrate;

(b) applying a thick film paste composition onto one or more insulating films to form a layered structure, wherein the thick film paste composition comprises:

i) 85-99.75 wt.% on a solids basis of a source of a conductive metal;

ii) 0.25 to 15 wt.% on solids of lead-tellurium-lithium-oxide; and

iii) an organic medium; and

(c) firing the semiconductor substrate, the one or more insulating films, and the thick film paste to form an electrode in contact with the one or more insulating layers and in electrical contact with the semiconductor substrate.

Another aspect of the invention is an article comprising:

a) a semiconductor substrate;

b) one or more insulating layers on the semiconductor substrate; and

c) an electrode in contact with one or more insulating layers and in electrical contact with the semiconductor substrate, the electrode

Comprising a conductive metal and a lead-tellurium-lithium-oxide.

Drawings

Fig. 1 is a process flow diagram illustrating a semiconductor device manufacturing process. The reference numerals shown in fig. 1 are explained as follows.

10: p-type silicon substrate

20: n-type diffusion layer

30: insulating film

40: p + layer (Back surface field, BSF)

60: aluminum paste deposited on the back side

61: aluminum back electrode (obtained by baking aluminum paste on the back side)

70: silver or silver/aluminium paste deposited on the back side

71: silver or silver/aluminum back electrode (obtained by baking back side silver paste)

500: thick film paste deposited on front side

501: front electrode (formed by firing thick film paste)

Detailed Description

Photovoltaic systems utilizing solar energy are considered environmentally friendly because they reduce the demand for fossil fuels.

The present invention provides compositions useful for making photovoltaic devices having improved electrical properties. The thick film paste composition comprises:

a) 85-99.75 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

b) 0.25-15 wt% lead-tellurium-lithium-oxide, based on solids; and

c) an organic medium.

As defined herein, the organic medium is not considered to be part of the solids comprising the thick film paste composition.

Conductive metal

The conductive metal is selected from the group consisting of silver, copper and palladium. The conductive metal may be in flake form, spherical form, granular form, crystalline form, powder or other irregular form, and mixtures thereof. The conductive metal may be provided in the form of a colloidal suspension.

When the metal is silver, it may be in the form of silver metal, silver derivatives or mixtures thereof. Exemplary derivatives include: for example, silver alloy, silver oxide (Ag)₂O), silver salts such as AgCl, AgNO₃、AgOOCCH₃(silver acetate), AgOOCF₃(silver trifluoroacetate), silver orthophosphate (Ag)₃PO₄). Other forms of silver that are compatible with other thick film paste components can also be used.

In one embodiment, the conductive metal or derivative thereof is from about 85 to about 99.75 weight percent of the solid component of the thick film paste composition. In another embodiment, the source of the conductive metal or derivative thereof is from about 90 to about 95 weight percent of the solid components of the thick film paste composition.

In one embodiment, the solid portion of the thick film paste composition comprises from about 85 to about 99.5 wt% spherical silver particles. In one embodiment, the solid portion of the thick film paste composition comprises from about 85 to about 90 weight percent silver particles and from about 1 to about 9.5 weight percent silver flakes.

In one embodiment, the thick film paste composition comprises conductive coated silver particles. Suitable coatings include phosphates and surfactants. Suitable surfactants include polyoxyethylene, polyethylene glycol, benzotriazole, poly (ethylene glycol) acetic acid, lauric acid, oleic acid, capric acid, myristic acid, linoleic acid, stearic acid, palmitic acid, stearates, palmitates, and mixtures thereof. The salt counter ion may be ammonium, sodium, potassium, and mixtures thereof.

The particle size of silver is not subject to any particular limitation. In one embodiment, the average particle size is from 0.5 to 10 microns; in another embodiment, the average particle size is from 1 to 5 microns.

As used herein, "particle size" or "D₅₀"is intended to mean" average particle size "; "average particle size" means 50% volume distribution particle size. The volume distribution particle size can be determined by a number of methods understood by those skilled in the art, including but not limited to laser diffraction and dispersion methods using a Microtrac particle size analyzer (Largo, FL).

Lead-tellurium-lithium-oxide compositions

One aspect of the present invention relates to a lead-tellurium-lithium-oxide (Pb-Te-Li-O) composition. In one embodiment, these compositions may be glass compositions. In another embodiment, these compositions may be crystalline, partially crystalline, amorphous, partially amorphous, or combinations thereof. In one embodiment, the Pb-Te-Li-O composition may comprise more than one glass composition. In one embodiment, the Pb-Te-Li-O composition may comprise a glass composition and an additional composition, such as a crystalline composition. The term "glass" or "glass composition" will be used herein to denote any combination of the amorphous and crystalline materials described above.

In one embodiment, the glass composition described herein comprises a lead-tellurium-lithium-oxide. The glass composition may also include additional components such as silicon, silver, tin, bismuth, aluminum, cerium, zirconium, sodium, vanadium, fluorine, niobium, sodium, tantalum, potassium, magnesium, phosphorus, selenium, cobalt, palladium, ruthenium, nickel, manganese, chromium, and the like.

PbO, TeO can be reacted using techniques understood by those of ordinary skill in the art₂And Li₂O (or other material that decomposes to the desired oxide upon heating) to produce lead-tellurium-lithium-oxide (Pb-Te-Li-O). Such preparation techniques may involve heating the mixture in air or an oxygen-containing atmosphere to form a melt, quenching the melt, and milling, and/or screening the quenched material to provide a powder having a desired particle size. Melting of the mixture of lead, tellurium and lithium oxides is typically carried out to a peak temperature of 800-. The molten mixture can be quenched, for example, on a stainless steel platen or between counter-rotating stainless steel rollers to form a sheet. The resulting flakes can be ground to form a powder. Typically, the milled powder has a D of 0.1 to 3.0 microns₅₀. Alternative synthesis techniques may be used by those skilled in the art of frit manufacturing, such as, but not limited to, water quenching, sol-gel, spray pyrolysis, or other synthesis techniques suitable for preparing glass in powder form.

In one embodiment, the starting mixture for preparing Pb-Te-Li-O may comprise (based on the weight of the total starting mixture):

PbO, which may be 30-60 wt%, 40-55 wt%, or 45-50 wt%;

TeO₂it may be 40-65 wt%, 45-60 wt% or 50-55 wt%; and

Li₂o, whichMay be 0.1-5 wt%, 0.2-3 wt%, or 0.3-1 wt%.

In another embodiment, in addition to PbO, TeO as described above₂And Li₂In addition to O, the starting mixture for the preparation of Pb-Te-Li-O may also comprise SiO₂、SnO₂、B₂O₃、Ag₂O、BiF₃,V₂O₅,Na₂O,ZrO₂,CeO₂,Bi₂O₃、Nb₂O₅、Ta₂O₅、K₂O、MgO、P₂O₅、SeO₂、Co₃O₄、PdO、RuO₂、NiO、MnO、Cr₂O₃Or Al₂O₃One or more of the above. In aspects of this embodiment (based on the weight of the total starting mixture):

SiO₂may be 0-11 wt%, 0-5 wt%, 0.25-4 wt% or 0-0.5 wt%;

SnO₂may be 0-5 wt%, 0-2 wt% or 0.5-1.5 wt%;

B₂O₃may be 0-10 wt%, 0-5 wt% or 0.5-5 wt%;

Ag₂o can be 0-30 wt%, 0-20 wt%, 3-15 wt%, or 1-8 wt%;

TiO₂may be 0-5 wt%, 0.25-5 wt% or 0.25-2.5 wt%;

PbF₂may be 0-20 wt%, 0-15 wt% or 5-10 wt%;

BiF₃may be 0-15 wt%, 0-10 wt% or 1-10 wt%;

ZnO can be 0-5 wt%, 0-3 wt%, or 2-3 wt%;

V₂O₅may be 0-5 wt%, 0-1 wt% or 0.5-1 wt%;

Na₂o can be 0-5 wt%, 0-3 wt% or 0.1-1.5 wt%；

CuO may be 0-5 wt%, 0-3 wt%, or 2-3 wt%;

ZrO₂may be 0-3 wt%, 0-2 wt% or 0.1-1 wt%;

CeO₂may be 0-5 wt%, 0-3 wt% or 0.1-2.5 wt%;

Bi₂O₃may be 0-15 wt%, 0-10 wt% or 5-8 wt%; and is

Al₂O₃May be 0-3 wt%, 0-2 wt% or 0.1-2 wt%.

In one embodiment, the Pb-Te-Li-O may be a uniform powder. In another embodiment, the Pb-Te-Li-O may be a combination of more than one powder, where each powder may be individually a uniform population. The composition of the overall combination of the two powders is within the ranges as described above. For example, Pb-Te-Li-O may comprise a combination of two or more different powders; separately, these powders may have different compositions and may or may not be within the ranges as described above; however, the combination of these powders may be in the range as described above.

In one embodiment, a Pb-Te-Li-O composition may include a first powder including a homogeneous powder containing some, but not all, of the Pb, Te, Li, and O group elements, and a second powder including one or more of the Pb, Te, Li, and O group elements.

In one embodiment, Li₂Some or all of O may be replaced by Na₂O、K₂O、Cs₂O or Rb₂O instead, a glass composition having properties similar to those listed above was obtained. In this embodiment, the total alkali metal oxide content may be 0 to 5 wt.%, 0.1 to 3 wt.%, or 0.25 to 3 wt.%.

In another embodiment, one or more of the glass frit compositions herein may comprise one or more of the components of the third group: GeO₂、Ga₂O₃、In₂O₃、NiO、CoO、ZnO、CaO、MgO、SrO、MnO、BaO、SeO₂、MoO₃、WO₃、Y₂O₃、As₂O₃、La₂O₃、Nd₂O₃、Bi₂O₃、Ta₂O₅、V₂O₅、FeO、HfO₂、Cr₂O₃、CdO、Sb₂O₃、PbF₂、ZrO₂、Mn₂O₃、P₂O₅、CuO、La₂O₃、Pr₂O₃、Nd₂O₃、Gd₂O₃、Sm₂O₃、Dy₂O₃、Eu₂O₃、Ho₂O₃、Yb₂O₃、Lu₂O₃、CeO₂、BiF₃、SnO、SiO₂、Ag₂O、Nb₂O₅、TiO₂And metal halides (e.g., NaCl, KBr, NaI, LiF).

Thus, as used herein, the term "Pb-Te-Li-O" may also comprise a metal oxide comprising one or more elements selected from: si, Sn, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, Bi, F, Zr, Mn, P, Cu, Ce and Nb.

Tables 1, 2 and 4 list the compositions comprising PbO, TeO₂、Li₂Some examples of powder mixtures of O and other optional compounds that can be used for the preparation of lead-tellurium-lithium-oxide. The list is intended to be illustrative, not limiting. In tables 1, 2 and 4, the amounts of the compounds are shown in weight percent based on the weight of the total glass composition.

Typically, PbO and TeO₂The mixture of powders comprises 5 to 95 mole% of lead oxide and 5 to 95 mole% of lead oxide, based on the mixed powdersTellurium oxide in mole%. In one embodiment, PbO and TeO₂The mixture of powders comprises 25-50 mole% lead oxide and 50-75 mole% tellurium oxide, based on the mixed powders.

The glass compositions described herein, also referred to as frits, contain certain components in certain percentages. Specifically, the percentages refer to the percentage of the components used within the raw materials that will subsequently be processed into a glass composition as described herein. Such nomenclature is conventional to those skilled in the art. In other words, the composition comprises certain components and the percentages of these components are expressed in terms of the corresponding percentages of the oxide forms. As known to those of ordinary skill in the art of glass chemistry, a certain portion of the volatile species may be released during the process of making the glass. An example of a volatile substance is oxygen.

If the starting material is a fired glass, one of ordinary skill in the art can calculate the percentages of the starting components described herein using methods known to those skilled in the art, including, but not limited to: inductively coupled plasma-emission spectroscopy (ICPES), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and the like. Further, the following exemplary techniques may be used: x-ray fluorescence spectroscopy (XRF), nuclear magnetic resonance spectroscopy (NMR), electron paramagnetic resonance spectroscopy (EPR), mossbauer spectroscopy, electron microprobe Energy Dispersive Spectroscopy (EDS), electron microprobe Wavelength Dispersive Spectroscopy (WDS), Cathodoluminescence (CL).

One of ordinary skill in the art will recognize that the raw materials selected may inadvertently contain impurities that may be incorporated into the glass during processing. For example, the impurities may be present in a range of hundreds to thousands of ppm.

The presence of impurities does not alter the characteristics of the glass, thick film composition or fired device. For example, a solar cell comprising the thick film composition can have the efficiencies described herein even if the thick film composition contains impurities.

Organic medium

The inorganic components of the thick film paste composition are mixed with an organic medium to form a viscous paste having a consistency and rheology suitable for printing. A variety of inert viscous materials can be used as the organic medium. The organic medium may be an organic medium in which the inorganic components may be dispersed with a sufficient degree of stability during manufacture, shipping and storage of the paste, as well as on the printing screen during the screen printing process.

Suitable organic media have rheological properties to provide stable dispersion of the solids, suitable viscosity and thixotropy for screen printing, suitable wettability of the substrate and paste solids, good drying rate, and good firing characteristics. The organic medium may comprise thickeners, stabilizers, surfactants, and/or other common additives. The organic medium may be a solution of one or more polymers in one or more solvents. Suitable polymers include ethyl cellulose, ethyl hydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins, polymethacrylates of lower alcohols, and the monobutyl ether of ethylene glycol monoacetate. Suitable solvents include terpenes such as alpha-or beta-terpineol or mixtures thereof with other solvents such as kerosene, dibutyl phthalate, butyl carbitol acetate, hexylene glycol and alcohols having a boiling point above 150 ℃ and alcohol esters. 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 epoxidised resinate, isotetradecanol, and pentaerythritol esters of hydrogenated rosin the organic medium can also contain a volatile liquid to promote rapid hardening after application of the thick film paste composition on a substrate.

The optimum amount of organic medium in the thick film paste composition depends on the method of applying the paste and the particular organic medium used. Typically, the thick film paste composition comprises 70-95 wt% inorganic components and 5-30 wt% organic medium.

If the organic medium comprises a polymer, the polymer will typically comprise from 8 to 15% by weight of the organic composition.

Preparation of Thick film paste compositions

In one embodiment, the thick film paste composition may be prepared by mixing the conductive metal powder, the Pb-Te-Li-O powder, and the organic medium in any order. In some embodiments, the inorganic materials are first mixed and then added to the organic medium. If desired, the viscosity can be adjusted by adding a solvent. Mixing methods that provide high shear may be useful.

Another aspect of the invention is a method comprising:

(a) providing an article comprising one or more insulating films deposited onto at least one surface of a semiconductor substrate;

(b) applying a thick film paste composition onto at least a portion of one or more insulating films to form a layered structure, wherein the thick film paste composition comprises:

i) 85-99.75 wt.% on a solids basis of a source of a conductive metal;

ii) 0.25 to 15 wt.% on solids of lead-tellurium-lithium-oxide; and

iii) an organic medium; and

(c) firing the semiconductor substrate, the one or more insulating films and the thick film paste, wherein the organic medium of the thick film paste is volatilized, thereby forming an electrode in contact with the one or more insulating layers and in electrical contact with the semiconductor substrate.

In one embodiment, the thick film paste may comprise lead-tellurium-lithium-oxide in an amount of 0.25 to 15 wt%, 0.5 to 7 wt%, or 1 to 3 wt% based on solids.

In one embodiment, a semiconductor device is made of an article including a semiconductor substrate bearing a junction and a silicon nitride insulating film formed on a main surface thereof. The method comprises the following steps: a thick film paste composition capable of penetrating the insulating layer is applied (e.g., coated or screen printed) in a predetermined shape and thickness to a predetermined position on the insulating film, and then fired so that the thick film paste composition reacts with and penetrates the insulating film to make electrical contact with the silicon substrate.

One embodiment of the process is shown in figure 1.

Fig. 1(a) shows a monocrystalline or polycrystalline silicon p-type substrate 10.

In fig. 1(b), an n-type diffusion layer 20 having an opposite polarity is formed to create a p-n junction. Using phosphorus oxychloride (POCl) as the phosphorus source₃) The n-type diffusion layer 20 is formed by thermal diffusion of phosphorus (P). Without any particular modification, the n-type diffusion layer 20 is formed over the entire surface of the silicon p-type substrate. The depth of the diffusion layer can be varied by controlling the diffusion temperature and time, and is typically formed in a thickness range of about 0.3-0.5 microns. The n-type diffusion layer may have a film resistivity of several tens of ohms/square up to at most about 120 ohms/square.

As shown in fig. 1(c), after one surface of this n-type diffusion layer 20 is protected with a resist or the like, the n-type diffusion layer 20 is removed from a plurality of surfaces by etching so that it remains on only one main surface. The resist is then removed using an organic solvent or the like.

Next, in fig. 1(d), an insulating layer 30, which also serves as an antireflection coating, is formed on the n-type diffusion layer 20. The insulating layer is typically silicon nitride, but may also be SiN_xAn H film (i.e., an insulating film containing hydrogen which plays a role in passivation in a subsequent firing process), a titanium oxide film, a silicon oxide film, a carbon-containing silicon nitride film, a carbon-containing silicon oxide film, a carbon-containing silicon oxynitride film, or a silicon oxide/titanium oxide film. AboutThe thickness of the silicon nitride film of (a) is suitable for a refractive index of about 1.9-2.0. The insulating layer 30 can be deposited by sputtering, chemical vapor depositionPhase deposition, or other methods.

Then, an electrode is formed. As shown in fig. 1(e), the thick film paste composition of the present invention is screen printed on the insulating film 30 and then dried. Further, an aluminum paste 60 and a back side silver paste 70 are screen-printed on the back side of the substrate, and are sequentially dried. Firing at a temperature of 750-850 ℃ for a time of several seconds to several tens of minutes.

Thus, as shown in fig. 1(f), during firing, aluminum diffuses from the aluminum paste into the silicon substrate on the back side, forming a p + layer 40 containing a high concentration of aluminum dopant. This layer is generally referred to as a Back Surface Field (BSF) layer and helps to improve the energy conversion efficiency of the solar cell. Firing converts the dried aluminum paste 60 into an aluminum back electrode 61. The back side silver paste 70 is simultaneously fired to become the silver or silver/aluminum back electrode 71. During firing, the boundary between the back side aluminum and the back side silver assumes an alloyed state, thereby achieving electrical connection. The aluminum electrode occupies a large area of the back electrode, due in part to the need to form the p + layer 40. Meanwhile, since it is impossible to solder the aluminum electrode, a silver or silver/aluminum back electrode is formed on a limited area of the back side as an electrode for interconnecting solar cells by a copper ribbon or the like.

On the front side, the thick film paste composition 500 of the present invention sinters and penetrates the insulating film 30 during firing to make electrical contact with the n-type diffusion layer 20. This type of process is commonly referred to as "burn-through". The fire-through state, i.e., the degree to which the paste melts and penetrates the insulating film 30, depends on the quality and thickness of the insulating film 30, the composition of the paste, and the firing conditions. When fired, the slurry 500 becomes an electrode 501, as shown in FIG. 1 (f).

In one embodiment, the insulating film is selected from titanium oxide, aluminum oxide, silicon nitride, SiN_xH, silicon oxide, carbon-doped silicon oxynitride, carbon-containing silicon nitride film, carbon-containing silicon oxide film, and silicon oxide/titanium oxide film. The silicon nitride film may be formed by sputtering, Plasma Enhanced Chemical Vapor Deposition (PECVD), or thermal chemical vapor deposition. In one embodiment, the silicon oxide film is formed by thermal oxidation, sputtering, or thermal chemical vapor deposition or plasma chemical vapor deposition. The titanium oxide film may be formed by coating an organic liquid material containing titanium onto a semiconductor substrate and baking, or by thermal chemical vapor deposition.

In one embodiment of the method, the semiconductor substrate may be monocrystalline or polycrystalline silicon.

Suitable insulating films comprise one or more components selected from: alumina, titanium oxide, silicon nitride, SiN_xH, silicon oxide, carbon-doped silicon oxynitride, carbon-containing silicon nitride film, carbon-containing silicon oxide film, and silicon oxide/titanium oxide. In one embodiment of the present invention, the insulating film is an antireflective coating (ARC). The insulating film may be applied to the semiconductor substrate, or it may be naturally formed, as in the case of silicon oxide.

In one embodiment, the insulating film comprises a layer of silicon nitride. Silicon nitride may 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 may be a chemical treatment. Removal of at least a portion of the silicon nitride can result in improved electrical contact between the thick film paste composition conductor and the semiconductor substrate. This may result in an improved efficiency of the semiconductor device.

In one embodiment, the silicon nitride of the insulating film is part of the antireflective coating.

The thick film paste composition may be printed on the insulating film in the form of a pattern (e.g., a bus bar having a connection line). The printing can be performed by screen printing, electroplating, extrusion, ink-jet, molding or multi-plate printing or ribbons.

During the electrode formation 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 such that the organic binder material from the dried thick film paste composition, as well as any other organic materials present, are burned out. In one embodiment, the firing temperature is 750-. Sintering can be carried out in a belt furnace using high transport rates (e.g., 100-600 cm/min) with final hold times of 0.05-5 minutes. Multiple temperature zones (e.g., 3-11 zones) can be used to control the desired heat distribution.

Upon firing, the conductive metal and the Pb-Te-Li-O mixture penetrate the insulating film. The penetration of the insulating film results in 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, a tellurium compound, lead, a lead compound, and a silicon compound, wherein the silicon may be derived from a silicon substrate and/or one or more insulating layers. After firing, the electrodes comprise a sintered metal that contacts the underlying semiconductor substrate, and may also contact one or more insulating layers.

Another aspect of the invention is an article formed by a method comprising:

(a) providing an article comprising one or more insulating films deposited onto at least one surface of a semiconductor substrate;

(b) applying a thick film paste composition onto at least a portion of one or more insulating films to form a layered structure, wherein the thick film paste composition comprises:

i) 85-99.75 wt.% on a solids basis of a source of a conductive metal;

ii) 0.25 to 15 wt.% on solids of lead-tellurium-lithium-oxide; and

iii) an organic medium, and

(c) firing the semiconductor substrate, the one or more insulating films and the thick film paste, wherein the organic medium of the thick film paste is volatilized, thereby forming an electrode in contact with the one or more insulating layers and in electrical contact with the semiconductor substrate.

Such articles can be used in the manufacture of photovoltaic devices. In one embodiment, the article is a semiconductor device comprising an electrode formed from a 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.

Examples

Exemplary preparation and evaluation of thick film paste compositions are described below.

Example I

Preparation of lead-tellurium-lithium-oxides

Preparation of lead-tellurium-lithium-oxides for glasses 1 to 7 in Table 1 and for glasses 2 and 3

By mixing and blending Pb₃O₄、TeO₂And Li₂CO₃Powder to prepare the lead-tellurium-lithium-oxide (Pb-Te-Li-O) compositions of table 1. The blended powder batch was loaded into a platinum alloy crucible and then inserted into a furnace at 900-₂Of the atmosphere (c). The heat treatment was continued for 20 minutes after the components had reached complete dissolution. The low viscosity liquid resulting from the component fusion is then quenched with a metal roller. The quenched glass is then ground and sieved to provide D₅₀Is 0.1-3.0 μm powder.

By mixing and blending Pb₃O₄、TeO₂And Li₂CO₃Powder and optionally (as shown in Table 2) SiO₂、Al₂O₃、ZrO₂、B₂O₃、Nb₂O₅、Na₂CO₃、Ta₂O₅、K₂CO₃、Ag₂O、AgNO₃、CeO₂And/or SnO₂To prepare the lead-tellurium-lithium-oxide (Pb-Te-Li-O) compositions of Table 2.

Preparation of lead-tellurium-lithium-oxides of glasses 8 to 14 of Table 1 and glasses in Table 4

Adding TeO₂Powder (99 + purity%), PbO powder and Li₂CO₃The mixture of powders (ACS reagent grade, 99+ purity%) was placed in a suitable container and rolled for 15-30 minutes to mix the starting powders of glass compositions 8-14 of table 1. For the compositions of Table 4, TeO is added₂PbO or Pb₃O₄And Li₂CO₃And optionally (as shown in Table 4) SiO₂、Bi₂O₃、BiF₃、SnO₂、Al₂O₃、MgO、Na₂O、Na₂CO₃、NaNO₃、P₂O₅Aluminum phosphate, lead phosphate, SeO₂、PbSeO₃、Co₃O₄、CoO、PdO、PdCO₃、Pd(NO₃)₂、RuO₂、ZrO₂、SiZrO₄、V₂O₅、NiO、Ni(NO₃)₂、NiCO₃、MnO、MnO₂、Mn₂O₃、Cr₂O₃The mixture of (a) is placed in a suitable container and rolled for 15-30 minutes to mix the starting powders. The starting powder mixture was placed in a platinum crucible and then heated to 900 ℃ in air at a heating rate of 10 ℃/min and then held at 900 ℃ for 1 hour to melt the mixture. The melt was quenched from 900 ℃ by removing the platinum crucible from the furnace and pouring the melt onto a stainless steel platen. The resulting material was placed in a mortar and ground to less than 100 mesh with a pestle. The milled material was then ball milled in a polyethylene container with zirconia balls and isopropanol until D₅₀Up to 0.5-0.7 micron. The ball-milled material was then separated from the milling balls, dried and passed through a 230 mesh screen to provide in the preparation of a thick film pasteFlux powder was used.

Table 1: glass frit composition in weight percent

Note that: the compositions in the table are expressed in weight percent based on the weight of the total glass composition. TeO₂The ratio of/PbO is only TeO in the composition₂And PbO.

Example II

Slurry preparation

Slurries preparation for the examples in tables 6, 7, 8, 9, 10 and 11

Generally, the slurry formulation was prepared using the following procedure: the appropriate amounts of solvent, binder, resin, and surfactant in table 5 were weighed and mixed in a mixing tank for 15 minutes to form an organic medium.

TABLE 5

Components	By weight%
		2,2, 4-trimethyl-1, 3-pentanediol monoisobutyrate	5.57
Ethyl cellulose (50-52% hydroxyethyl)	0.14
		Ethyl cellulose (48-50% hydroxyethyl)	0.04
N-tallow-1, 3-diaminopropane dioleate	1.00
		Hydrogenated castor oil	0.50
Pentaerythritol tetraesters of perhydroabietic acid	1.25
		Adipic acid dimethyl ester	3.15
Glutaric acid dimethyl ester	0.35

Since silver is the major component in solids, it is added incrementally to the media to ensure better wetting. After thorough mixing, the slurry was repeatedly crushed using a three-roll mill and the pressure was gradually increased from 0 to 250 psi. The gap of the rolls was set to 2 mils. The viscosity of the slurry was measured using a brookfield viscometer, and appropriate amounts of solvent and resin were added to adjust the viscosity of the slurry towards a target value between 230 and 280 Pa-s. The degree of dispersion is measured by the fineness of grind (FOG). For the fourth length of continuous draw down, the typical FOG value for the slurry is less than 20 microns, and when 50% of the slurry is drawn down, the FOG value is less than 10 microns.

In order to obtain the data in tables 6, 7, 8, 9, 10 and 11 for the final pastes used, 2-3 wt% of the glass frits in table 1 were mixed into a portion of the silver paste and dispersed by shearing between rotating glass plates used as a mill known to those skilled in the art. Alternatively, two separate slurries were prepared as follows: 1) rolling an appropriate amount of Ag with the media in table 5; and 2) rolling an appropriate amount of the glass frit of Table 1 with the media of Table 5. Appropriate amounts of silver paste and glass frit were then mixed together using a planetary centrifugal mixer (Thinky Corporation, Tokyo, Japan) to form the tested pastes.

The slurry examples of tables 6, 7, 8, 9, 10 and 11 were prepared using the procedure described above for preparing the slurry compositions listed in the tables in accordance with the following details. The tested pastes contained 85-88% silver powder. Example Using D₅₀Spherical silver of =2.0 μm.

To prepare the final slurries of tables 10 and 11, three separate slurries were prepared by the following steps: 1) adding an appropriate amount of silver to an appropriate amount of the carrier in table 5 and performing roll milling, 2) adding an appropriate amount of the 1 st glass frit from table 1 to an appropriate amount of the carrier in table 5 and performing roll milling, and 3) adding an appropriate amount of the 2 nd glass frit from table 2 to an appropriate amount of the carrier in table 5 and performing roll milling. Appropriate amounts of silver paste and glass frit were mixed together using a planetary centrifugal mixer (Thinky Corporation (Tokyo, Japan)) as indicated by the paste compositions in tables 10 and 11.

Table 3 shows the mixed frit compositions of the examples in tables 10 and 11. The mixed glass frit compositions shown in table 3 were calculated using the glass frit compositions of tables 1 and 2 at the blending ratios of tables 10 and 11.

Preparation of thick film pastes for the examples in tables 12, 13, 14 and 15

The organic components and relative amounts of the thick film pastes are given in table 5.

The organic components (-4.6 g) were placed into a Thinky mixing jar (Thinky USA, Inc.) and the Thinky mixing continued at 2000RPM for 2-4 minutes until good blending was achieved. The inorganic components (Pb-Te-Li-O powder and silver conductive powder) were put into a glass jar and subjected to rolling mixing for 15 minutes. The total weight of the inorganic components was 44g, of which 42.5 to 43.5g was silver powder, and 0.5 to 1.5g was Pb-Te-Li-O powder of Table 1. One third of the inorganic components was then added to the Thinky jar with the organic components and mixed at 2000RPM for 1 minute. This operation was repeated until all the inorganic components had been added and mixed. The slurry was allowed to cool and the viscosity was adjusted to between 200 and 500Pa · s by adding solvent and then mixing at 2000RPM for 1 minute. This procedure was repeated until the desired viscosity was obtained. The slurry was then roller milled three times at 0psi and three times at 75psi with a 1 mil gap. The degree of dispersion is measured by the fineness of grind (FOG). For thick film pastes, the FOG value is typically equal to or less than 20/10. After 24 hours the viscosity of the slurry was adjusted to between 200 and 320Pa · s at room temperature. After 3 minutes the viscosity was measured with a viscometer at 10 RPM. The viscosity of each slurry was measured using a Brookfield viscometer (Brookfield, inc., (Middleboro, MA)) using a 14-spindle and a 6-cup.

Example III

Solar cell preparation

Solar cell preparation of the examples listed in tables 6, 7, 8, 9, 10 and 11

The slurries from examples 1 and 13-22 were applied to DeutscheCell (DeutscheCell, Germany) polycrystalline wafers having 65 Ω/□ emitters, and the slurries from examples #2 to #6 were applied to Gintech (Gintech Energy Corporation, (Taiwan)) polycrystalline wafers having 55 Ω/□ emittersThe solar cell used is textured by isotropic acid etching and has SiN_XH an anti-reflective coating (ARC). Efficiency and fill factor were measured for each sample as shown in tables 6, 7, 8, 9, 10 and 11. For each slurry, the mean and median values of efficiency and fill factor are shown for 5-10 samples. Each sample was screen printed using an ETPL555 type printer with a doctor blade speed set at 250 mm/s. The screen used had the following pattern on a 20 μm emulsion in a screen with 325 mesh and 23 μm wires: 11 finger lines with 100 μm openings and 1 busbar with 1.5mm openings. Commercially available aluminum paste DuPont PV381 was printed on the non-light-receiving (back) side of the device.

The device with the printed pattern on both sides was then placed in a drying oven and dried at a peak temperature of 250 ℃ for 10 minutes. Then, the substrate was fired in an IR furnace in the CF7214Despatch 6 zone with the light side up, the IR furnace using a band speed of 560cm/min and a temperature set point of 550-600-650-800-905-945 ℃. The actual temperature of the component is measured during the process. The estimated peak temperature for each part was 740-780 ℃ and each part was above 650 ℃ for a total time of 4 seconds. The fully processed samples were then tested for PV performance using a calibrated ST-1000 tester.

Solar cell fabrication of the examples of tables 12, 13, 14 and 15

Solar cells for testing thick film paste performance were made from 200 micron DeutscheCell (DeutscheCell, Germany) multicrystalline silicon wafers with 65 Ω/sq. phosphorus doped emitter layer with acid etched textured surface and 70nm thick PECVD SiN_xAfter the wafer was diced, it was screen printed using an AMI-Presco MSP-485 screen printer to provide 1 busbar, 11 wires spaced 0.254cm apart, and a screen printed aluminum back side conductor that was completely groundedFiring the firing temperatures shown in tables 12, 13, 14 and 15 are the furnace set point temperatures for the final peak region, which are about 125 ℃ higher than the actual wafer temperature the fired wire had a median line width of 120 microns and an average line height of 15 microns the median line resistivity was 3.0 × 10^-6Omega cm. anticipates that the performance of a 28mm × 28mm cell will be affected by edge effects to reduce the overall solar cell Fill Factor (FF) by about-5%.

Example IV

Solar cell performance: efficiency and fill factor

And (3) testing procedures: efficiency and fill factor of tables 6, 7, 8, 9, 10 and 11

Solar cells constructed according to the methods described herein were tested for conversion efficiency. Exemplary efficiency testing methods are provided below.

In one embodiment, a solar cell constructed according to the methods described herein is placed in a commercial I-V tester (Telecom STV, model ST-1000) for measuring efficiency. The xenon arc lamp in the I-V tester simulates sunlight of known intensity AM1.5 and illuminates the front of the cell. The tester measures the current (I) and voltage (V) at approximately 400 load resistance settings using a multi-point contact method to determine the current-voltage curve of the battery. The Fill Factor (FF) and efficiency (Eff) are both calculated from the current-voltage curves.

Solar cell electrical measurements of the examples of tables 12, 13, 14 and 15

The solar cell performance of the examples of tables 12 and 13 was measured at 25 ℃ ± 1.0 ℃ using a ST-1000 type IV tester from Telecom STV co. The xenon arc lamp in the IV tester simulates sunlight of known intensity and illuminates the front of the cell. The tester measures the current (I) and voltage (V) at a load resistance set point of about 400 using a four-point contact method to determine the current-voltage curve of the cell. The solar cell efficiency (Eff), Fill Factor (FF), and series resistance (Rs) were calculated from the current-voltage curves (data for Rs not shown).

The median and average values of the efficiencies and fill factors for these examples are shown in tables 12, 13, 14, and 15.

Comparative example I: bismuth-tellurium-lithium oxide

Preparation of bismuth-tellurium-lithium oxides

Using boron oxide (B)₂O₃) Zinc oxide (ZnO), titanium oxide (TiO)₂) Bismuth oxide (Bi)₂O₃) Tellurium oxide (TeO)₂) Lithium carbonate (LiCO)₃) And lithium phosphate (LiPO)₄) And by the above example I: the procedures described in the preparation of the lead-tellurium-lithium-oxide of glasses 1 to 7 in Table 1 and the glasses in tables 2 and 3 prepare compositions as shown in Table 16 comprising bismuth-tellurium-lithium-oxide (Bi-Te-Li-O).

Table 16: bismuth-tellurium-lithium-oxide compositions in weight percent on oxide basis

	Glass A (wt%)
			2.09
ZnO	0.98
			0.48
	26.64
			67.22
	0.43
			2.16

Note that: the compositions in the table are shown in weight percent based on the weight of the total glass composition

Slurry preparation

The paste using glass a was prepared by the following procedure. A paste was prepared by mixing appropriate amounts of the organic vehicle (table 5) and silver powder. The silver paste was passed through a three roll mill and the pressure was gradually increased from 0 to 75 psi. The viscosity of the silver paste was measured using a brookfield viscometer, and appropriate amounts of solvent and resin were added to adjust the viscosity of the paste towards a target value between 230 and 280 Pa-s. Another slurry was prepared by mixing the appropriate amounts of organic vehicle (table 5) and glass frit a. The glass frit was passed through a three-roll mill and the pressure was gradually increased from 0 to 250 psi. The degree of dispersion of each slurry was measured by the fineness of grind (FOG). For the fourth length of continuous draw down, the typical FOG value for the slurry is less than 20 microns, and when 50% of the slurry is drawn down, the FOG value is less than 10 microns.

The silver paste and glass frit slurry were mixed together using a planetary centrifugal mixer (Thinky Corporation (Tokyo, Japan)) to make the final slurry formulations shown in table 17.

Solar cell preparation and efficiency and fill factor measurement

Slurry is applied to a 1.1 '× 1.1.1' dicing saw cut polycrystalline silicon solar cell with phosphorus doped emitters on the p-type substrate the slurry is applied to a DeutscheCell (DeutscheCell, Germany) polycrystalline wafer with 62 Ω/□ emitters the solar cell used is textured by isotropic acid etching and has SiN_XH an anti-reflective coating (ARC). The efficiency and fill factor of each sample were measured as shown in table 17. Each sample was prepared by screen printing using an ETP L555 type printer with a doctor blade speed set at 200 mm/s. The screen used had the following pattern on a 20 μm emulsion in a screen with 325 mesh and 23 μm wires: 11 finger lines with 100 μm openings and 1 busbar with 1.5mm openings. Will have a businessCommercially available aluminum paste DuPont PV381 was printed on the non-light-receiving (back) side of the apparatus.

The device with the printed pattern on both sides was then placed in a drying oven and dried at a peak temperature of 250 ℃ for 10 minutes. The substrate was then fired with the light side up in a CF7214Despatch 6 zone infrared furnace using a belt speed of 560cm/min and temperature set points of 500-. The actual temperature of the component is measured during the process. The estimated peak temperature for each part was 745 ℃ 775 ℃, and each part was above 650 ℃ for a total time of 4 seconds. The fully processed samples were then tested for PV performance using a calibrated ST-1000 tester.

The efficiency and fill factor for each sample were measured as shown in table 17. For each slurry, the mean and median of the efficiency and fill factor for 6 samples are shown.

Claims (14)

1. A thick film paste composition for forming an electrical connection in a photovoltaic device comprising at least one insulating layer on a major surface thereof, said composition comprising:

a) 85-99.75 wt% silver, a silver derivative selected from the group consisting of silver alloys, silver oxides, and silver salts, or mixtures thereof, based on total solids in the composition;

b) 0.25 to 15 wt.% on solids basis of lead-tellurium-lithium-oxide comprising 0.1 to 5 wt.% of Li₂O; and

c) an organic medium;

the thick film paste is capable of penetrating the at least one insulating layer after firing.

2. The thick-film paste composition of claim 1, wherein the molar ratio of lead to tellurium in the lead-tellurium-lithium-oxide is between 5/95 and 95/5.

3. The thick-film paste composition of claim 1, wherein the lead-tellurium-lithium-oxide comprises:

30-60 wt% of PbO,

40-65 wt.% of TeO₂And are and

0.1-5 wt% of Li₂O；

The total weight of all components in the lead-tellurium-lithium-oxide is 100%.

4. The thick-film paste composition of claim 1, wherein said organic medium comprises a polymer.

5. The thick-film paste composition of claim 4, wherein said organic medium further comprises one or more additives selected from the group consisting of solvents, stabilizers, surfactants, and thickeners.

6. The thick film paste composition of claim 1, wherein said silver, silver derivative or mixture thereof is 90-95 wt% of said solids.

7. The thick-film paste composition of claim 1, wherein said lead-tellurium-lithium-oxide is at least partially crystalline.

8. The thick-film paste composition of claim 3, wherein said lead-tellurium-lithium-oxide further comprises one or more selected from the group consisting of: GeO₂、Ga₂O₃、In₂O₃、NiO、CoO、ZnO、CaO、MgO、SrO、MnO、BaO、SeO₂、MoO₃、WO₃、Y₂O₃、As₂O₃、La₂O₃、Nd₂O₃、Bi₂O₃、Ta₂O₅、V₂O₅、FeO、HfO₂、Cr₂O₃、CdO、Sb₂O₃、PbF₂、ZrO₂、Mn₂O₃、P₂O₅、CuO、La₂O₃、Pr₂O₃、Al₂O₃、Nd₂O₃、Gd₂O₃、Sm₂O₃、Dy₂O₃、Eu₂O₃、Ho₂O₃、Yb₂O₃、Lu₂O₃、CeO₂、BiF₃、SnO、SiO₂、Ag₂O、Nb₂O₅、TiO₂And a metal halide selected from the group consisting of: NaCl, KBr, NaI, and LiF.

9. The thick-film paste composition of claim 1, wherein said lead-tellurium-lithium-oxide further comprises an oxide of one or more elements selected from the group consisting of: si, Sn, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La, Nd, Co, Pr, Al, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, Bi, F, Zr, Mn, P, Cu, Ce and Nb.

10. A method of forming an electrode, comprising:

(a) providing a semiconductor substrate comprising one or more insulating films deposited onto at least one surface of the semiconductor substrate;

(b) applying a thick film paste composition onto at least a portion of the insulating film to form a layered structure, wherein the thick film paste composition comprises:

i) 85-99.75 wt% silver, a silver derivative selected from the group consisting of silver alloys, silver oxides, and silver salts, or mixtures thereof, based on total solids in the composition;

ii) 0.25 to 15 wt.%, based on solids, of a lead-tellurium-lithium-oxide comprising 0.1 to 5 wt.% of Li₂O; and

iii) an organic medium; and

(c) firing the semiconductor substrate, one or more insulating films and thick film paste, wherein the organic medium in the thick film paste is volatilized, thereby forming an electrode in contact with the one or more insulating layers and in electrical contact with the semiconductor substrate.

11. The method of claim 10, wherein the thick film paste composition is applied onto the insulating film in a pattern.

12. The method of claim 10, wherein the firing is performed in air or an oxygen-containing atmosphere.

13. The method of claim 10, wherein the lead-tellurium-lithium-oxide comprises:

30-60 wt% of PbO,

40-65 wt.% of TeO₂And are and

0.1-5 wt% of Li₂O；

The total weight of all components in the lead-tellurium-lithium-oxide is 100%.

14. A solar cell, comprising:

(a) a semiconductor substrate;

(b) one or more insulating layers on the semiconductor substrate; and

(c) an electrode in contact with the one or more insulating layers and in electrical contact with the semiconductor substrate, the electrode formed from the thick film paste composition of any one of claims 1-9.