HK1181743A - Thick-film pastes containing lead- and tellurium-oxides, and their use in the manufacture of semiconductor devices - Google Patents

Description

Thick film pastes containing lead and tellurium oxides and their use in semiconductor device fabrication

Technical Field

The 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 conductive metal and a lead-tellurium-oxide dispersed in an organic medium.

Background

Conventional solar cell structures with p-type substrates have a negative electrode, typically on the front side (illumination 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 generation of charge carriers of hole-electron 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 paste (also referred to as ink) is commonly used to form conductive grids or metal contacts. Conductive pastes generally include a glass frit, a conductive material (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 contacts are prepared between the grid lines and the semiconductor substrate.

However, crystalline silicon PV cells are typically coated with an antireflective coating such as silicon nitride, titanium oxide or silicon oxide to promote light absorption, which increases cell efficiency. Such antireflective coatings also serve as insulators that reduce the flow of electrons from the substrate to the metal contacts. To overcome this problem, the conductive ink should penetrate the antireflective coating during firing to form the metal contacts that are in electrical contact with the semiconductor substrate. It is also desirable to form a strong bond between the metal contact and the substrate.

The ability to penetrate the antireflective coating and form a strong bond with the substrate upon firing is highly dependent on the conductive ink composition and firing conditions. The efficiency, a key measure of PV cell performance, is also affected by the quality of the electrical contact made between the fired conductive ink and the substrate.

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

Summary of The Invention

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

a) 85-99.5 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

b) 0.5-15 wt.% lead-tellurium-oxide, based on solids, wherein the molar ratio of lead to tellurium in the lead-tellurium-oxide is between 5/95 and 95/5; 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 thereon;

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

wherein the thick film paste composition comprises:

i) 85-99.5 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

ii) 0.5-15 wt.% on solids of lead-tellurium-oxide, wherein the molar ratio of lead to tellurium in the lead-tellurium-oxide is between 5/95 and 95/5; 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 present 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 the one or more insulating layers and in electrical contact with the semiconductor substrate, the electrode comprising a conductive metal and a lead-tellurium-oxide.

Brief Description of 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

Solar photovoltaic systems are considered environmentally friendly because they reduce the demand for fossil fuels.

The present invention provides compositions that can be used to make photovoltaic devices with improved electrical properties. The thick film paste composition comprises:

a) 85-99.5 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

b) 0.5-15 wt.% lead-tellurium-oxide, based on solids, wherein the molar ratio of lead to tellurium in the lead-tellurium-oxide is between 5/95 and 95/5; and

c) an organic medium.

As defined herein, the organic medium is not considered part of a solid (including a 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 the form of flakes, spheres, granules, crystals, powders, or other irregular forms and mixtures thereof. The conductive metal may be provided in a colloidal suspension.

When the metal is silver, it can be silver metalSilver derivatives, or mixtures thereof. Exemplary derivatives include: such as silver alloy, silver oxide (Ag)₂O), silver salts, e.g. AgCl, AgNO₃、AgOOCCH₃(silver acetate), AgOOCF₃(silver trifluoroacetate), or Ag₃PO₄(silver orthophosphate). Other forms of silver that are compatible with other thick film paste components may also be used.

In one embodiment, the conductive metal or derivative thereof is about 85 to about 99.5 weight percent of the solid components of the thick film paste composition. In another embodiment, the conductive metal or derivative thereof is 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% of 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 electrically conductive coated silver particles. Suitable coatings comprise phosphate and surfactant. 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 can 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 laser diffraction and dispersion method using a Microtrac particle size analyzer.

Lead-tellurium-oxides

The lead-tellurium-oxide (Pb-Te-O) can be prepared by: adding TeO₂Mixing with lead oxide powder, heating the powder mixture under air or an oxygen-containing atmosphere to form a melt, quenching the melt, milling and ball milling the quenched material, and screening the milled material to provide a powder having a desired particle size. The lead oxide powder may include one or more components selected from the group consisting of: PbO, Pb₃O₄And PbO₂. The mixture of lead oxide and tellurium oxide is typically fired to a peak temperature of 800-. The molten mixture may 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 milled to form a powder. Typically, the milled powder has a D of 0.1 to 3.0 microns₅₀. In one embodiment, the Pb-Te-O formed in this manner may be at least partially crystalline.

Typically, PbO and TeO based on the mixed powder₂The mixture of powders comprises 5-95mol% of lead oxide and 5-95mol% of tellurium oxide. In one embodiment, PbO and TeO are based on the mixed powder₂The mixture of powders comprises 30-85mol% of lead oxide and 15-70mol% of tellurium oxide. In another embodiment, PbO and TeO are based on the mixed powder₂The mixture of powders comprises 30-65mol% of lead oxide and 35-70mol% of tellurium oxide.

In some embodiments, PbO and TeO₂The mixture of powders also comprises one or more other metal compounds. Suitable other metal compounds include TiO₂、LiO₂、B₂O₃、PbF₂、SiO₂、Na₂O、K₂O、Rb₂O、Cs₂O、Al₂O₃、MgO、CaO、SrO、BaO、V₂O₅、ZrO₂、MoO₃、Mn₂O₃、Ag₂O、ZnO、Ga₂O₃、GeO₂、In₂O₃、SnO₂、Sb₂O₃、Bi₂O₃、BiF₃、P₂O₅、CuO、NiO、Cr₂O₃、Fe₂O₃、CoO、Co₂O₃And CeO₂. Tables 1 and 2 list the compositions comprising PbO, TeO₂And other optional metal compounds that may be used to make the lead-tellurium oxide. This list is intended to be illustrative and not limiting.

Thus, as used herein, the term "Pb-Te-O" may also include metal oxides comprising oxides of one or more elements selected from the group consisting of: si, Sn, Li, 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.

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: the inorganic components can be dispersed in the organic medium with a suitable degree of stability during manufacture, shipment and storage of the paste, and can be dispersed on the printing screen during the screen printing process.

Suitable organic media have rheological properties that 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 and alcohol esters having a boiling point above 150 ℃. 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, isodecyl alcohol and pentaerythritol esters of hydrogenated rosins the organic medium may also include a volatile liquid to promote rapid hardening of the thick film paste composition after application on a substrate.

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

If the organic medium comprises a polymer, the organic composition may comprise from 8 to 15 wt% of the polymer.

Preparation of Thick film paste compositions

In one embodiment, the thick-film paste composition can be prepared by mixing the conductive metal powder, Pb-Te-O powder, and 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 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 one or more insulating films to form a layered structure, wherein the thick-film paste composition comprises:

i) 85-99.5 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

ii) 0.5-15 wt.% on solids of lead-tellurium-oxide, wherein the molar ratio of lead to tellurium in the lead-tellurium-oxide is between 5/95 and 95/5; 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.

In one embodiment, a semiconductor device is fabricated from an article comprising a semiconductor substrate bearing a junction and a silicon nitride insulating film formed on a major 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) to the insulating film in a predetermined shape and thickness and at a predetermined location, and then fired so that the thick film paste composition reacts with and penetrates the insulating film for the purpose of making electrical contact with the silicon substrate.

One embodiment of the method is shown in figure i.

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

In fig. l (b), an n-type diffusion layer 20 of reverse polarization is formed to create a p-n junction. The n-type diffusion layer 20 can be formed by using phosphorus oxychloride (POCl)₃) Thermal diffusion of phosphorus (P) as a phosphorus source. Without any particular modification, the n-type diffusion layer 20 is formed on 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 O.3-O.5 microns. The n-type diffusion layer may have a film resistivity of several tens of ohms/square.

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

Next, in fig. l (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_x: an H film (i.e., an insulating film containing hydrogen for passivation during a subsequent firing process), a titanium oxide film, or a silicon oxide film. AboutThe silicon nitride film thickness of (a) is suitable for a refractive index of about 1.9-2. O. The deposition of the insulating layer 30 may be by sputtering, chemical vapor deposition, or other methods.

Then, an electrode is formed. As shown in FIG. l (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. The firing is carried out at a temperature of 750-.

Thus, as shown in fig. l (f), during firing, aluminum diffuses from the aluminum paste into the silicon substrate on the back side, thereby 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 6 l. At the same time, the silver paste 70 on the back side is fired to become a silver or silver/aluminum back electrode 7 l. 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. At the same time, since soldering of the aluminum electrode is not possible, 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 means of copper tape 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 and thus achieves an electrical contact with the n-type diffusion layer 20. This type of process is generally referred to as "burn-through". The fire-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 the firing conditions. As shown in fig. 1 (f), when fired, the paste 500 becomes an electrode 501.

In one embodiment, the insulating film is selected from titanium oxide, aluminum oxide, silicon nitride, SiN_xH, silicon oxide, silicon carbon oxynitride, silicon nitride film containing carbon, silicon oxide film containing carbon, 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 methods. In one embodiment, the silicon oxide film is formed by thermal oxidation, sputtering, or thermal CFD or plasma CFD. The titanium oxide film may be formed by coating a titanium-containing organic liquid material onto a semiconductor substrate and baking, or by thermal chemical vapor deposition.

In this method, the semiconductor substrate may be a single crystal or polycrystalline silicon electrode.

Suitable insulating films include one or more components selected from the group consisting of: alumina, titanium oxide, silicon nitride, SiN_xH, silicon oxide, silicon carbon oxynitride, silicon nitride film containing carbon, silicon oxide film containing carbon, 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 occurring, as in the case of silicon oxide.

In one embodiment, the insulating film includes a silicon nitride layer. 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 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 the antireflective coating.

The thick film paste composition can be printed on the insulating film in a pattern, such as a bus bar with connecting lines. The printing may be by screen printing, electroplating, extrusion, ink-jetting, molding or multi-plate printing or tape printing.

During this electrode formation process, the thick film paste composition is heated to remove the organic medium and sinter the metal powder. The heating may 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, is burned out. In one embodiment, the firing temperature is 750-. The firing may be carried out in a belt furnace using a high transfer rate, e.g., 100 and 500cm/min, with a final hold time of 0.05 to 5 minutes. Multiple temperature zones, such as 3-11 zones, may be used to control the desired heat distribution.

Upon firing, the conductive metal and lead-Te-O mixture penetrate the insulating film. The penetration of the insulating film achieves an electrical contact between the electrode and the semiconductor substrate. After firing, an interlayer can 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 can originate from the silicon substrate and/or one or more insulating layers. After firing, the electrode comprises 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 a semiconductor substrate comprising one or more insulating films deposited onto at least one surface of the semiconductor substrate;

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

wherein the thick film paste composition comprises:

i) 85-99.5 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

ii) 0.5-15 wt.% on solids of lead-tellurium-oxide, wherein the molar ratio of lead to tellurium in the lead-tellurium-oxide is between 5/95 and 95/5; 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.

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

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

Example I

Preparation of lead-tellurium-oxides

Preparation of glass frit lead-tellurium-oxides of tables 1 and 2

TeO₂Powder (99 +% purity) and PbO powder (ACS reagent grade, 99+% purity) and optionally PbF₂、SiO₂、B₂O₃、P₂O₅Lead phosphate, SnO2, SnO, Li₂O、Li2(CO₃)、Li(NO₃)、V₂O₅、Ag₂O、Ag₂(CO₃) The mixture of Ag (NO3) was tumbled in a polyethylene container for 30 minutes to mix the starting powders. The starting powder mixture was placed in a platinum crucible and heated to 900 ℃ in air at a heating rate of 10 ℃/min and then held at 900 ℃ for one 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 ground to stucco and pounded to less than 100 mesh with a pestle. The milled material was then ball milled in a polyethylene vessel with zirconia balls and isopropanol until D₅₀Is 0.5-0.7 μm. The ball-milled material was then separated from the milling balls, dried and run through a 100 mesh screen to provide the flux powder for thick film paste preparation.

Table 1: illustrative examples of powder mixtures that can be used to prepare suitable lead-tellurium oxides。

Table 2: glass frit composition in weight%

Note that: the compositions in the tables are expressed in weight percent based on the weight of the total glass composition. TeO₂A ratio of/PbO of between TeO only of the composition₂And PbO.

Lead-tellurium-oxide preparation of the glass frits of table 3

The lead-tellurium-lithium-oxide (Pb-Te-Li-Ti-O) compositions of Table 3 were prepared by mixing and blending Pb₃O₄And TeO₂Powder, and optionally SiO as shown in Table 3₂、P₂O₅、Pb₂P₂O₇、Ag₂O、Ag(NO₃) And/or SnO₂To prepare the compound. The blended powder batch is loaded into a platinum alloy crucible and then placed in a crucible using air or containing O₂An atmosphere in a furnace at 900-. The duration of the heat treatment is 20 minutes after complete melting of the components has been reached. The resulting low viscosity liquid resulting from the melting of the components is then quenched with a metal roller. The quenched glass is then ground and screened to provide a glass having D₅₀Is 0.1 to 3.0 micron powder.

Table 3: glass frit composition in weight%

Glass #	SiO2	PbO	P2O5	Ag2O	SnO2	TeO2
							27		44.53		7.71		47.76
28				59.22		40.78
							29		41.72		13.54		44.75
30		80.75				19.25
							31	1.66	41.85	0.86	9.58	1.16	44.89
32		58.31				41.69
							33	5.95	54.27		5.41	123	33.14

Note that: the compositions in the tables are expressed in weight percent based on the weight of the total glass composition.

Example II

Paste preparation

Preparation of Thick film pastes for tables 5, 6, 7 and 8

The organic components and relative amounts of the thick film paste are given in table 2.

Table 4: organic component of thick film paste

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

The organic components were placed in a Thinky mixing jar (Thinky USA, Inc) and Thinky-mixed at 2000RPM for 2-4min until fully blended. The inorganic components (Pb-Te-O powder and silver conductive powder) were tumble-mixed in a glass jar for 15 min. The total weight of the inorganic components is 88g, wherein 85-87g is silver powder and 1-3g is PbO and TeO₂A mixture of powders. One third of the inorganic components was then added to the Thinky jar containing the organic components and mixed at 2000RPM for 1 min. This procedure was repeated until all the inorganic components were added and mixed. The paste was cooled and the viscosity adjusted to between 200 and 500Pa · s by adding solvent, then mixed for 1min at 200 RPM. This procedure is repeated until the desired viscosity is obtained. The paste was then rolled 3 times at zero psi with a 1 mil gap and rolled 3 times at 75 psi. The degree of dispersion is measured by the fineness of grind (FOG). For thick film pastes, the grind fineness value is typically equal to or less than 20/10. The viscosity of each paste was measured on a brookfield viscometer with a #14 spindle and a #6 cup. After 24 hours, the viscosity of the paste was adjusted to between 200 and 320Pa · s at room temperature. The viscosity was measured after 3 minutes in a viscometer at 10 RPM.

Table 5: thick film paste composition

Preparation of Thick film pastes for tables 9, 10, 12 and 13

Generally, paste formulations were prepared using the following procedure: the appropriate amounts of solvent, medium and surfactant from tables 9, 10, 12 and 13 were weighed and mixed in the mixing tank for 15 minutes.

Since silver is the major component in the solid, it is added in stepwise increments to ensure better wetting. After thorough mixing, the paste was repeatedly rolled with a three-roll mill and the pressure was gradually increased from 0 to 250 psi. The gap of the rolls was set at 2 mils. The viscosity of the paste was measured using a brookfield viscometer and appropriate amounts of solvent and resin were added to adjust the paste viscosity to a target viscosity between 230 and 280 Pa-sec. The degree of dispersion is measured by the fineness of grind (FOG). For the fourth longest continuous draw down, the paste typically has a grind size value of less than 20 microns. When 50% of the paste was scraped, the grind fineness value was less than 10 microns.

To prepare the final pastes used to generate the data in tables 9, 10, 12 and 13, 2-3 wt% of the glass frit from table 1 was mixed into a portion of the silver paste and dispersed by shearing between rotating glass plates known to those skilled in the art as a mill. To prepare the final pastes of tables 12 and 13, three separate pastes were prepared by the following steps: 1) an amount of silver was added to an amount of the roll-milled table 4 carrier, 2) an amount of the 1 st glass frit from table 3 was added to an amount of the roll-milled table 4 carrier, and 3) an amount of the 2 nd glass frit from table 3 was added to an amount of the roll-milled table 4 carrier. Appropriate amounts of silver paste and frit paste were mixed together using a planetary centrifugal mixer (Thinky Corporation, Tokyo, Japan).

Table 11 shows the glass frit compositions of the examples of tables 12 and 13 mixed. The mixed frit compositions shown in table 11 were calculated using the frit compositions of table 3 at the blending ratios of tables 12 and 13.

The paste examples of tables 5, 7, 8, 9, 12 and 13 were prepared using the above procedure for preparing the paste compositions listed in the tables in accordance with the following details. The paste tested contained 85-88% silver powder. These examples use a catalyst having D₅₀Single spherical silver of =2.0 μm.

Example III

Solar cell preparation

Preparation of solar cells in tables 6, 7 and 8

Solar cells for testing thick film paste performance were made from 175 micron thick q.cell poly silicon wafers with 65 Ω/sq phosphorus doped emitter layer with acid etched textured surface and 70nm thick PECVDSiN_xAnd (4) antireflection coating. The solar cell is supplied by Q-CellsSE (OT Thalheim, Germany). The wafers were cut into 28mm x 28mm wafers using a diamond dicing saw. After the wafer was diced, it was screen printed using an AMI-Presco MSP-485 screen printer to provide busbars, 11 conductor lines with a pitch of 0.254cm and a full ground plane, screen printed aluminum back side conductor. After printing and drying, the cells were fired in a BTU international rapid thermal processing belt furnace. The firing temperature shown in table 3 is the furnace set point temperature, which is about 125 ℃ greater 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 resistivity of the median line is 3.0E-6ohm cm. The performance of a 28mm x 28mm cell is expected to be affected by edge effects to reduce the overall solar cell Fill Factor (FF) by-5%.

Solar cell preparation of the examples in tables 9, 10, 12 and 13

The paste was applied to a 1.1 "x 1.1" dicing saw cut polycrystalline silicon solar cell with a phosphorus doped emitter on a p-type substrate. The paste from example #1 was applied to a polycrystalline wafer having a 62 Ω/□ emitter of deutsche cell (deutsche cell, Germany), and the pastes from examples #2 to #6 were used to a polycrystalline wafer having a 55 Ω/□ emitter of Gintech (Gintech Energy Corporation, Taiwan). The solar cell used is isotropically acid-etched textured and has SiN_XAn anti-reflective coating (ARC) of H. For each sample, the efficiency and fill factor as shown in tables 9, 10, 12 and 13 were measured. For each paste, the mean and median values of efficiency and fill factor for 5-12 samples are shown. ETP L555 type stamp using a doctor blade set at 250mm/secEach sample was prepared by screen printing with a brush machine. 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 side (back side) of the device.

The device with the printed pattern on both sides was then dried in a drying oven at a peak temperature of 250 ℃ for 10 minutes. Then, the substrate was fired with the light irradiation side up in a CF7214 Despatch 6 zone infrared 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 part is measured during the process. The estimated peak temperature for each part was 740-. The finished samples were then tested for PV performance using a calibrated ST-1000 tester.

Example IV

Solar cell performance: efficiency and fill factor

And (3) testing procedures: efficiency and fill factor for the examples of tables 6, 7 and 8

Solar cell performance was measured using an ST-1000, TelecomSTV Co.IV tester at 25 ℃ +/-1.0 ℃. The xenon arc lamp in the IV tester simulates sunlight with a known intensity and illuminates the front side of the cell. The tester utilizes a four-point contact method to measure the current (I) and voltage (V) at a load resistance set point of about 400 to determine the current-voltage curve of the cell. Solar cell efficiency (Eff), Fill Factor (FF), and series resistance (Rs) were calculated from the current-voltage curve. Rs is affected by, among other things, the contact resistivity (ρ c), the conductor linear resistance, and the emitter sheet resistance. Since the conductor linear resistance and sheet resistance are nominally equal for different embodiments, the difference in Rs is primarily due to ρ c. The ideality factor is determined using the Suns-VOC technique. The ideality factor is reported as 0.1 solar radiance.

The median values of efficiency, fill factor, series resistance and ideality factor for solar cells prepared using the thick film pastes of examples 1-12 were determined and summarized in table 6. The average and median values of the efficiencies for solar cells prepared using the thick film pastes of examples 13-27 were determined and summarized in table 7. The mean and median fill factors for solar cells prepared using the thick film pastes of examples 13-27 were determined and summarized in Table 8.

Table 6: properties of paste

And (3) testing procedures: efficiency and fill factor for the examples of tables 9, 10, 12 and 13

Solar cells formed 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 (TelecomSTV, model ST-1000) for measuring efficiency. Sunlight was simulated with a xenon arc lamp of known intensity AM 1.5 in an I-V tester and the cell front was illuminated. The tester measures the current (I) and voltage (V) at a load resistance setting of about 400 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.

Table 11: mixing from the blending frit experiments of tables 12 and 13 using the frits of table 3 Synthetic frit compositions

Blended glass composition numbering	PbO	Ag2O	TeO2
				I	33.40	20.58	46.02
II	37.70	20.92	41.37
				III	29.15	29.61	41.24
IV	43.73	14.80	41.47

Comparative example I: bismuth-tellurium-oxide

Preparation of bismuth-tellurium-oxides

Bismuth-tellurium-oxide (Bi-Te-O) comprising the composition as shown in Table 14 uses 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 prepared by the method described in example I above: lead-tellurium-oxide preparation of the glass frits of table 3

Table 14: bismuth-tellurium-oxide compositions expressed as% by weight of the oxide

Note that: the compositions in the tables are expressed in weight percent based on the weight of the total glass composition.

Paste preparation

The paste using glass a was prepared by the following method. The paste was prepared by mixing the appropriate equivalents of organic vehicle (Table 4) and silver powder. The silver paste was rolled with 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 paste viscosity to a target viscosity between 230 and 280 Pa-sec. Another paste was prepared by mixing appropriate equivalents of organic vehicle (Table 4) and glass frit A. The glass frit slurry is passed through a three-roll mill at gradually increasing pressures of 0 to 250 psi. The degree of dispersion of each paste was measured by fineness of grind (FOG). For the fourth longest continuous draw down, the paste typically has a grind size value of less than 20 microns. When 50% of the paste was scraped, the grind fineness value was less than 10 microns.

The silver and glass frit slurries were mixed together using a planetary centrifugal mixer (Thinky Corporation, Tokyo, Japan) to prepare the final paste formulations shown in table 15.

Solar cell preparation and efficiency and fill factor measurement

The paste was applied to a 1.1 "x 1.1" dicing saw cut polycrystalline silicon solar cell with a phosphorous doped emitter on a p-type substrate. The paste was applied to a DeutscheCell (DeutscheCell, Germany) polycrystalline wafer with a 62 Ω/□ emitter. The solar cell used is textured by isotropic acid etching and has SiN_XAn anti-reflective coating (ARC) of H. The efficiency and fill factor as shown in table 15 were measured for each sample. Each sample was prepared by screen printing using an ETP L555 type printer with a doctor blade speed set at 200 mm/sec. 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 side (back side) of the device.

The device with the printed pattern on both sides was then dried in a drying oven at a peak temperature of 250 ℃ for 10 minutes. The substrate was then fired with the light side up in a CF7214 Despatch 6 zone infrared furnace using a band speed of 560cm/min and a temperature set point of 500-550-610-700-HZ 6, where HZ6=885, 900 and 915 ℃. The actual temperature of the part is measured during the process. The estimated peak temperature for each part is 745-775 deg.C and the temperature for each part is above 650 deg.C for a total time of 4 seconds. The finished samples were then tested for PV performance using a calibrated ST-1000 tester.

For each sample, the efficiency and fill factor shown in table 15 were measured. For each paste, the mean and median of the efficiency and fill factor for 6 samples are shown.

Claims (14)

1. A thick film paste composition comprising:

a) 85-99.5 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

b) 0.5-15 wt.% lead-tellurium-oxide, based on solids, wherein the molar ratio of lead to tellurium in the lead-tellurium-oxide is between 5/95 and 95/5; and

c) an organic medium.

2. The thick-film paste of claim 1, wherein the conductive metal comprises silver.

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

4. The thick-film paste of claim 3, wherein said organic medium further comprises one or more additives selected from the group consisting of: solvents, stabilizers, surfactants, and thickeners.

5. The thick-film paste of claim 1, wherein the electrically conductive metal is 90-95 wt% of the solids.

6. The thick-film paste of claim 1, wherein said Pb-Te-O is at least partially crystalline.

7. The thick-film paste of claim 1, further comprising an additive selected from the group consisting of: TiO 2₂、LiO₂、B₂O₃、PbF₂、SiO₂、Na₂O、K₂O、Rb₂O、Cs₂O、Al₂O₃、MgO、CaO、SrO、BaO、V₂O₅、ZrO₂、MoO₃、Mn₂O₃、Ag₂O、ZnO、Ga₂O₃、GeO₂、In₂O₃、SnO₂、Sb₂O₃、Bi₂O₃、BiF₃、P₂O₅、CuO、NiO、Cr₂O₃、Fe₂O₃、CoO、Co₂O₃And CeO₂。

8. The thick-film paste of claim 1, wherein the lead-tellurium oxide further comprises one or more elements selected from the group consisting of: si, Sn, Li, 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.

9. The method comprises the following steps:

(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) 85-99.5 wt% of a conductive metal or derivative thereof, based on total solids in the composition;

ii) 0.5-15 wt.% on solids of lead-tellurium-oxide, wherein the molar ratio of lead to tellurium in the lead-tellurium-oxide is between 5/95 and 95/5; and

iii) an organic medium; and

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

10. The method of claim 9, wherein the thick film paste composition is applied onto the insulating film in a pattern.

11. The method of claim 9, wherein the firing is performed in air or an oxygen-containing atmosphere.

12. An article of manufacture, 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 comprising a conductive metal and a lead-tellurium-oxide.

13. The article of claim 12, wherein the article is a semiconductor device.

14. The article of claim 13, wherein the semiconductor device is a solar cell.