WO2015089188A1

WO2015089188A1 - Acrylic resin-containing organic vehicle for electroconductive paste

Info

Publication number: WO2015089188A1
Application number: PCT/US2014/069553
Authority: WO
Inventors: David C. Kapp; George T. SMITH; Gregory M. Berube; Cuiwen Guo; Lin Jiang; Krupali PATEL
Original assignee: Heraeus Precious Metals North America Conshohocken Llc
Priority date: 2013-12-11
Filing date: 2014-12-10
Publication date: 2015-06-18
Also published as: TW201529655A

Abstract

An electroconductive paste composition for manufacturing a solar cell including conductive metallic particles, glass frit, and an organic vehicle which includes an organic solvent and an acrylic resin lacking an active hydrogen functionality, is provided. The electroconductive paste composition is capable of being screen printed through screen openings having a diameter of about 40 µm or less.

Description

ACRYLIC RESIN-CONTAINING ORGANIC VEHICLE

FOR ELECTROCONDUCTIVE PASTE

RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Application Serial No. 61/914,596, filed December 11, 2013, the disclosure of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] This invention relates to electroconductive pastes containing an organic vehicle, which includes an acrylic resin. These pastes are useful in the manufacture of solar cells. BACKGROUND

[0003] Solar cells are devices that convert the energy of light into electricity using the photovoltaic effect. Solar power is an attractive green energy source because it is sustainable and produces only non-polluting by-products. In operation, when light hits a solar cell, a fraction of the incident light is reflected by the surface and the remainder is transmitted into the solar cell. The photons of the transmitted light are absorbed by the solar cell, which is usually made of a semiconducting material such as silicon. The energy from the absorbed photons excites electrons of the semiconducting material from their atoms, generating electron-hole pairs. These electron-hole pairs are then separated by p-n junctions and collected by conductive electrodes which are applied on the solar cell surface. In this way, electricity may be conducted between interconnected solar cells.

[0004] Solar cells typically have electroconductive pastes applied to both their front and back surfaces which, when fired, form electrodes. A front side paste is typically screen printed onto the front side of the substrate to serve as a front electrode. A typical electroconductive paste contains metallic particles, glass frit, and an organic vehicle. These components must be carefully selected to take full advantage of the potential of the resulting solar cell. For example, the composition of the organic vehicle may have an impact on the performance of the resulting solar cell. The organic vehicle has an effect on the viscosity of the paste, thus affecting its printability. One problem associated with known front side paste compositions is that they are not easily printable through screen openings with a diameter below about 40 μm. When the paste is not easily printable, the resulting printed lines which form the electrode may not be uniform, thereby affecting the overall efficiency of the solar cells produced.

[0005] Chinese Patent Application Publication No. 101834008 discloses a self crosslinking acrylate resin for use in an electronic silver paste. The paste contains molecules with functional groups (e.g., acrylates, epoxides) which crosslink slowly at room temperature, thus increasing the viscosity of the paste and reducing storage stability. Such a resin would not be suitable for use in higher-temperature application pastes, because the thermally accelerated crosslinking would increase the organic molecular weight and inhibit clean burnout of the organic phase.

[0006] Japanese Patent Application Publication No. 2009/259826 discloses a black, electrically conductive paste for application to a plasma display panel. This paste includes an acrylic ester polymer resin, but also includes black pigment which is not suitable for formation of solar cell electrodes.

[0007] Accordingly, there is a need for improved electroconductive pastes having a suitable viscosity for screen printing through screen openings having a diameter of about 40 μm or less and which are suitable for solar cells.

SUMMARY

[0008] One embodiment of the invention relates to an electroconductive paste composition for manufacturing a solar cell. The paste comprises conductive metallic particles, glass frit, and an organic vehicle which includes (i) an organic solvent and (ii) an acrylic resin lacking an active hydrogen functionality.

[0009] The electroconductive paste of the invention is suitable for screen printing through screen openings having a diameter of about 40 μm or less, allowing for fine line printing while still providing adequate electrical conductivity.

[0010] Another embodiment of the invention relates to a method of producing a solar cell, comprising the steps of providing a silicon wafer having a front side and a backside, applying an electroconductive paste of the invention to the silicon wafer, and firing the silicon wafer. DETAILED DESCRIPTION

[0011] The pastes of the invention may be used to form electrodes on a solar cell. The electrodes provide the path by which conductivity occurs between solar cells. The electroconductive paste composition preferably includes conductive metallic particles, glass frit, and an organic vehicle including an acrylic resin. Organic Vehicle

[0012] A desired organic vehicle is one which allows for fine line printability, while also improving the physical characteristics of the printed line. Specifically, electroconductive pastes which may be screen printed and which form uniform (or substantially uniform) printed lines are preferred. One way to characterize the uniformity of the printed line is by its line definition, which can be determined by analyzing the longitudinal edge uniformity (i.e., variance in width along the length of the line) and variation in height (i.e.,“peaks” and“valleys” along the top of the line). The standard deviation in the line edge or height is used to determine, generally, the line uniformity. The lower the standard deviation, considering the line width and line height values, the better the line uniformity.

[0013] Such uniformity becomes compromised when screen printing current pastes through screen openings having a diameter of about 40 μm or less. With such narrow screen openings, the viscosity of the paste must be reduced (as compared to pastes which are printed through larger screen openings) in order to facilitate the deposit of the paste through the screen. Because the viscosity is reduced, the line definition is deteriorated, as the paste is more likely to“spread” along the surface of the substrate. Similarly, the aspect ratio (ratio between height and width) is also decreased. With decreased line uniformity and decreased aspect ratio, the efficiency of solar cell deteriorates. Typically, an aspect ratio above about 0.25 is preferable.

[0014] It is beneficial to print with relatively small screen openings (i.e., 40 μm or less) so as to produce finer lines, thereby covering less of the silicon wafer surface. By covering less of the surface with electroconductive paste, more of the silicon surface is exposed to sunlight which increases the efficiency of the solar cell. The invention permits printing of thin lines while maintaining line uniformity and aspect ratio.

[0015] In a preferred embodiment, the paste has a viscosity of at least about 50 Kcps and at most about 300 Kcps (kilocentipoise). Without being bound by any particular theory, it is believed that this viscosity range allows for optimal screen printing through screen openings having a diameter of about 40 μm or less.

[0016] In one embodiment, the electroconductive paste includes at least about 1 wt% organic vehicle, and more preferably at least about 5 wt% organic vehicle, based upon 100% total weight of the paste. At the same time, the electroconductive paste preferably includes no more than about 20 wt% organic vehicle, and more preferably no more than about 15 wt% organic vehicle, based upon 100% total weight of the paste.

[0017] According to a preferred embodiment, the organic vehicle comprises an acrylic resin. Current electroconductive paste compositions often use cellulose or cellulose ester as the resin component of the organic vehicle. Such pastes, however, do not print well through screen openings having a diameter of about 40 μm or less. The use of the acrylic resin in place of, or in addition to cellulose or cellulose ester improves printability using screens having a diameter of 40 μm or less, such that the uniformity of the printed line is improved.

[0018] The organic vehicle may comprise at least about 1 wt% of acrylic resin, preferably at least about 2 wt%, and most preferably at least about 4 wt%, based upon 100% total weight of the organic vehicle. At the same time, the organic vehicle preferably includes no more than about 15 wt% acrylic resin, preferably no more than 12% and most preferably no more than 10%, based upon 100% total weight of the organic vehicle. [0019] The acrylic resin preferably lacks an active hydrogen functionality (i.e., a hydrogen atom which can be donated, accepted, or shared). For example, the acrylic resin preferably lacks a hydroxyl, carboxyl or amide group (other than the carboxy group in the acrylate center). In one embodiment, the acrylic resin may be a polymer of an alkyl methacrylate, such as a C₁-C₆ methacrylate. The alkyl group may be linear or branched. For example, the resin may be a polymer of ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, or n-hexyl methacrylate. In another embodiment, the resin may be any other acrylate resin which lacks active hydrogen functionality, including, but not limited to, butyl acrylate or isobutyl acrylate. According to one embodiment, the acrylic resin does not self-crosslink.

[0020] The acrylic resin preferably has a molecular weight of at least about 50 kilodaltons (KD), and preferably at least about 100 KD. At the same time, the acrylic resin preferably has a molecular weight of no more than about 350 KD, preferably no more than about 300 KD, and most preferably no more than about 250 KD. A preferred acrylic resin is an isobutyl methacrylate polymer, for example, having a molecular weight of about 100-250 KD (for example, 193 KD).

[0021] According to one embodiment, the organic vehicle does not include a urethane acrylate resin.

[0022] According to another embodiment, the organic vehicle further comprises of an organic binder and an organic solvent. In a preferred embodiment, the organic vehicle also includes at least one of a surfactant or a thixotropic agent, or any combination thereof.

[0023] The solvent component is preferably removed from the paste to a significant extent during firing. In one embodiment, solvents which are present after firing with an absolute weight reduced by at least about 80%, more preferably by at least 95%, compared to before firing are preferred. Preferred solvents are those which provide the paste with improved viscosity, printability, stability and sintering characteristics. All solvents which are known in the art, and which are considered to be suitable in the context of this invention, may be employed as the solvent in the organic vehicle. Preferred solvents are those which exist as a liquid under standard ambient temperature and pressure (SATP) (298.15 K, 25°C, 77°F), 100 kPa (14.504 psi, 0.986 atm), more preferred are those with a boiling point above about 90°C and a melting point above about 20°C. [0024] Preferred solvents according to the invention include, but are not limited to, polar or non-polar, protic or aprotic, aromatic or non-aromatic compounds, and may be mono-alcohols, di-alcohols, poly-alcohols, mono-esters, di-esters, poly-esters, mono-ethers, di-ethers, poly- ethers, solvents which comprise at least one or more of these categories of functional groups, optionally comprising other categories of functional groups, preferably cyclic groups, aromatic groups, unsaturated bonds, alcohol groups with one or more O atoms replaced by heteroatoms (such as N atoms), ether groups with one or more O atoms replaced by heteroatoms (such as N atoms), esters groups with one or more O atoms replaced by heteroatoms (such as N atoms), and mixtures of two or more of the aforementioned solvents. Preferred esters in this context include, but are not limited to, di-alkyl esters of adipic acid, preferred alkyl constituents including methyl, ethyl, propyl, butyl, pentyl, hexyl and higher alkyl groups or combinations of two different such alkyl groups, preferably dimethyladipate, and mixtures of two or more adipate esters. Preferred ethers in this context include, but are not limited to, diethers, such as dialkyl ethers of ethylene glycol and mixtures of two diethers. The alkyl constituents in the dialkyl ethers of ethylene can be, for example, methyl, ethyl, propyl, butyl, pentyl, hexyl and higher alkyl groups or combinations of two different such alkyl groups. Preferred alcohols in this context include, but are not limited to, primary, secondary and tertiary alcohols, preferably tertiary alcohols, terpineol and its derivatives being preferred, or a mixture of two or more alcohols. Preferred solvents which combine more than one functional group include, but are not limited to, (i) 2,2,4- trimethyl-1,3-pentanediol monoisobutyrate, often called texanol, and its derivatives, (ii) 2-(2- ethoxyethoxy)ethanol, also known as carbitol, its alkyl derivatives, preferably methyl, ethyl, propyl, butyl, pentyl, and hexyl carbitol, preferably hexyl carbitol or butyl carbitol, and acetate derivatives thereof, preferably butyl carbitol acetate, or (iii) mixtures of at least two of the aforementioned.

[0025] In one embodiment, the solvent includes diethylene glycol mono butyl ether, diethylene glycol mono butyl ether acetate, or any combination thereof.

[0026] The organic vehicle may include at least about 60 wt% organic solvent, preferably at least about 70 wt%, and most preferably at least about 80 wt%, based upon 100% total weight of the organic vehicle. At the same time, the organic vehicle preferably includes no more than about 90 wt% organic solvent, based upon 100% total weight of the organic vehicle. [0027] According to another embodiment, the organic vehicle also includes a surfactant. Preferred surfactants in the context of the invention are those which contribute to the formation of an electroconductive paste with favorable stability, printability, viscosity and sintering properties. Preferred surfactants include, but are not limited to, those surfactants based on linear chains, branched chains, aromatic chains, fluorinated chains, siloxane chains, polyether chains and combinations thereof, and may be single chained, double chained or poly chained. Preferred surfactants may have non-ionic, anionic, cationic, amphiphilic, or zwitterionic heads. Preferred surfactants include, but are not limited to, polymeric surfactants, monomeric surfactants, and mixtures thereof. Preferred surfactants may have pigment affinic groups, preferably hydroxyfunctional carboxylic acid esters with pigment affinic groups (e.g., DISPERBYK®-108, manufactured by BYK USA, Inc.), acrylate copolymers with pigment affinic groups (e.g., DISPERBYK®-116, manufactured by BYK USA, Inc.), modified polyethers with pigment affinic groups (e.g., TEGO® DISPERS 655, manufactured by Evonik Tego Chemie GmbH), or other surfactants with groups of high pigment affinity (e.g., TEGO® DISPERS 662 C, manufactured by Evonik Tego Chemie GmbH). Other preferred surfactants include, but are not limited to, polyethylene oxide, polyethylene glycol and its derivatives, and alkyl carboxylic acids and their derivatives or salts, or mixtures thereof. The preferred polyethylene glycol derivative is poly(ethyleneglycol)acetic acid. Preferred alkyl carboxylic acids are those with fully saturated and those with singly or polyunsaturated alkyl chains or mixtures thereof. Preferred carboxylic acids with saturated alkyl chains are those with alkyl chain lengths in a range of from about 8 to about 20 carbon atoms, preferably C₉H₁₉COOH (capric acid), C₁₁H₂₃COOH (lauric acid), C₁₃H₂₇COOH (myristic acid), C₁₅H₃₁COOH (palmitic acid), C₁₇H₃₅COOH (stearic acid), or salts or mixtures thereof. Preferred carboxylic acids with unsaturated alkyl chains are C₁₈H₃₄O₂ (oleic acid) and C₁₈H₃₂O₂ (linoleic acid). A preferred combination comprising alkyl carboxylic acids in this context is castor oil. A preferred monomeric surfactant is benzotriazole and its derivatives.

[0028] The surfactant may be present in the organic vehicle in an amount of at least about 0.01 wt%. At the same time, the surfactant is preferably no more than about 10 wt%, preferably no more than about 5 wt%, and more preferably no more than about 3 wt%, based upon 100% total weight of the organic vehicle.

[0029] The organic vehicle may also comprise a thixotropic agent. Any thixotropic agent known to one having ordinary skill in the art may be used with the organic vehicle of the invention. For example, without limitation, thixotropic agents may be derived from natural origin, e.g., castor oil, or they may be synthesized. Preferred thixotropic agents are carboxylic acid derivatives, preferably fatty acid derivatives or combinations thereof. Commercially available thixotropic agents, such as, for example, Thixotrol^® MAX, may also be used. According to a preferred embodiment, the organic vehicle comprises at least about 5 wt% thixotropic agent, and preferably at least about 8 wt%, based upon 100% total weight of the organic vehicle. At the same time, the organic vehicle preferably includes no more than about 15 wt% thixotropic agent, preferably no more than about 13 wt%, based upon 100% total weight of the organic vehicle.

[0030] The organic vehicle may also comprise one or more additives. Preferred additives in the organic vehicle are those additives which are distinct from the aforementioned vehicle components and which contribute to favorable viscosity and printability of the electroconductive paste. Preferred additives include, but are not limited to, viscosity regulators, stabilizing agents, inorganic additives, thickeners, emulsifiers, dispersants or pH regulators. Preferred surfactants are carboxylic acids with saturated alkyl chains, such as those with alkyl chain lengths in a range of from about 8 to about 20 carbon atoms, preferably C9H19COOH (capric acid), C11H23COOH (lauric acid), C13H27COOH (myristic acid), C15H31COOH (palmitic acid), C17H35COOH (stearic acid), or salts or mixtures thereof. Preferred carboxylic acids with unsaturated alkyl chains are C18H34O2 (oleic acid) and C18H32O2 (linoleic acid). Conductive Metallic Particles

[0031] Conductive metallic particles in the context of the invention are those which exhibit optimal conductivity and which effectively sinter upon firing, such that they yield electrodes with high conductivity. Conductive metallic particles known in the art suitable for use in forming solar cell electrodes are preferred. Preferred metallic particles include, but are not limited to, elemental metals, alloys, metal derivatives, mixtures of at least two metals, mixtures of at least two alloys or mixtures of at least one metal with at least one alloy.

[0032] The electroconductive paste may comprise at least 35 wt% metallic particles, preferably at least 50 wt%, more preferably at least 70 wt%, and most preferably at least 80 wt%, based upon 100% total weight of the paste. At the same time, the electroconductive paste preferably includes no more than about 99 wt% metallic particles, preferably no more than about 95 wt%, based upon 100% total weight of the paste. Electroconductive pastes having a metallic particle content below 35 wt% may not provide sufficient electrical conductivity and adhesion, while electroconductive pastes having a metallic particle content above 95 wt% may have a viscosity which is too high for suitable screen printing.

[0033] Metals which may be employed as the metallic particles include at least one of silver, copper, gold, aluminum, nickel, and any mixtures or alloys of at least two thereof. In a preferred embodiment, the metallic particles are silver. The silver may be present as elemental silver, a silver alloy, or silver derivate. Suitable silver derivatives include, for example, silver alloys and/or silver salts, such as silver halides (e.g., silver chloride), silver oxide, silver nitrate, silver acetate, silver trifluoroacetate, silver orthophosphate, and combinations thereof. In another embodiment, the metallic particles may comprise a metal or alloy coated with one or more different metals or alloys, for example silver particles coated with aluminum or copper particles coated with silver.

[0034] The metallic particles may be present with a surface coating, either organic or inorganic. Any such coating known in the art, and which is considered to be suitable in the context of the invention, may be employed on the metallic particles. Preferred organic coatings are those coatings which promote dispersion into the organic vehicle. Preferred inorganic coatings are those coatings which regulate sintering and promote adhesive performance of the resulting electroconductive paste. If such a coating is present, it is preferred that the coating correspond to no more than about 5 wt%, preferably no more than about 2 wt%, and most preferably no more than about 1 wt%, based on 100% total weight of the metallic particles.

[0035] The conductive particles can exhibit a variety of shapes, surfaces, sizes, and surface area to volume ratios. Some examples of shapes include, but are not limited to, spherical, angular, elongated (rod or needle like) and flat (sheet like). Conductive metallic particles may also be present as a combination of particles with different shapes. One way to characterize such shapes is through the following parameters: length, width and thickness. In the context of the invention, the length of a particle is given by the length of the longest spatial displacement vector, both endpoints of which are contained within the particle. The width of a particle is given by the length of the longest spatial displacement vector perpendicular to the length vector defined above both endpoints of which are contained within the particle. The thickness of a particle is given by the length of the longest spatial displacement vector perpendicular to both the length vector and the width vector, both defined above, both endpoints of which are contained within the particle.

[0036] In one embodiment, metallic particles with shapes as uniform as possible are preferred (i.e. shapes in which the ratios relating any two of the length, the width and the thickness are as close as possible to 1; preferably at least 0.7, more preferably at least 0.8, and most preferably at least 0.9, and preferably no more than about 1.5, preferably no more than about 1.3, and most preferably no more than about 1.2). Examples of preferred shapes for the metallic particles in this embodiment are spheres and cubes, or combinations thereof, or combinations of one or more thereof with other shapes. In another embodiment, metallic particles are preferred which have a shape of low uniformity, preferably with at least one of the ratios relating the dimensions of length, width and thickness being above about 1.5, more preferably above about 3 and most preferably above about 5. Preferred shapes according to this embodiment are flake shaped, rod or needle shaped, or a combination of flake shaped, rod or needle shaped with other shapes.

[0037] Another parameter characterizing particle size distribution is d₅₀. The d₅₀ is the median diameter or the medium value of the particle size distribution. It is the value of the particle diameter at 50% in the cumulative distribution. Particle size distribution may be measured via laser diffraction, dynamic light scattering, imaging, electrophoretic light scattering, or any other methods known in the art. Specifically, particle size according to the invention is determined in accordance with ISO 13317-3:2001. A Microtrac S3500 instrument with accompanying software (manufactured by Microtrac, Inc. of Montgomeryville, Pennsylvania), which operates according to X-ray gravitational technique, is used for the measurement. A sample of about 0.3 grams is weighed into a 100 ml glass beaker and 2.3 grams of surfactant solution (5% RV-260 in deionized water) is added. The beaker is then filled to the 20 ml line with deionized water and stirred by hand. The beaker is placed on a stir play at 350 rpm for approximately one minute. Using a pipette, the sample is slowly loaded into the circulating fluid in the sample delivery controller and the measurement is started. The particle size distribution is determined by the software and given as d₅₀.

[0038] It is preferred according to the invention that the median particle diameter d₅₀ of the metallic particles be at least about 0.1 μm, and preferably at least about 0.5 μm. At the same time, the d₅₀ is preferably no more than about 5 μm, and more preferably no more than about 3.5 μm.

[0039] Another way to characterize the shape and surface of a particle is by its surface area to volume ratio. The surface area to volume ratio, or specific surface area, may be measured by the BET (Brunauer-Emmett-Teller) method, which is known in the art. Specifically, BET measurements are made in accordance with DIN ISO 9277:1995. A Monosorb instrument (manufactured by Quantachrome Instruments), which operates according to the SMART method (Sorption Method with Adaptive dosing Rate), is used for the measurement. According to one embodiment, the metallic particles may have a specific surface area of at least about 0.1 m²/g, preferably at least about 0.2 m²/g. At the same time, the specific surface area is preferably no more than 5 m²/g, and more preferably no more than about 2 m²/g. Glass Frit

[0040] The glass frit acts as an adhesion media, facilitating the bonding between the conductive particles and the silicon substrate, and thus providing reliable electrical contact. Specifically, the glass frit etches through the surface layers (e.g., antireflective layer) of the silicon substrate, such that effective electrical contact can be made between the electroconductive paste and the silicon wafer.

[0041] According to one embodiment, the electroconductive paste includes at least about 0.5 wt% glass frit, and preferably at least about 1 wt%, based upon 100% total weight of the paste. At the same time, the paste preferably includes no more than about 15 wt% glass frit, preferably no more than about 10 wt%, and most preferably no more than about 6 wt%, based upon 100% total weight of the electroconductive paste.

[0042] Preferred glass frits are powders of amorphous or partially crystalline solids which exhibit a glass transition. The glass transition temperature T_g is the temperature at which an amorphous substance transforms from a rigid solid to a partially mobile undercooled melt upon heating. Methods for the determination of the glass transition temperature are well known to the person skilled in the art. Specifically, the glass transition temperature T_g may be determined using a DSC apparatus SDT Q600 (commercially available from TA Instruments) which simultaneously records differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) curves. The instrument is equipped with a horizontal balance and furnace with a platinum/platinum-rhodium (type R) thermocouple. The sample holders used are aluminum oxide ceramic crucibles with a capacity of about 40-90 μl. For the measurements and data evaluation, the measurement software Q Advantage; Thermal Advantage Release 5.4.0 and Universal Analysis 2000, version 4.5A Build 4.5.0.5 is applied respectively. As pan for reference and sample, aluminum oxide pan having a volume of about 85 μl is used. An amount of about 10-50 mg of the sample is weighted into the sample pan with an accuracy of 0.01 mg. The empty reference pan and the sample pan are placed in the apparatus, the oven is closed and the measurement started. A heating rate of 10 K/min is employed from a starting temperature of 25 °C to an end temperature of 1000 °C. The balance in the instrument is always purged with nitrogen (N₂ 5.0) and the oven is purged with synthetic air (80% N₂ and 20% O₂ from Linde) with a flow rate of 50 ml/min. The first step in the DSC signal is evaluated as glass transition using the software described above, and the determined onset value is taken as the temperature for T_g.

[0043] Preferably, the T_g is below the desired firing temperature of the electroconductive paste. According to the invention, preferred glass frits have a T_g of at least about 200°C, and preferably at least about 250°C. At the same time, preferred glass frits have a T_g of no more than about 700°C, preferably no more than about 650°C, and most preferably no more than about 500°C.

[0044] The glass frit may also include elements, oxides, compounds which generate oxides upon heating, and/or mixtures thereof. According to one embodiment, the glass frit is lead-based and may include lead oxide or other lead-based compounds including, but not limited to, salts of lead halides, lead chalcogenides, lead carbonate, lead sulfate, lead phosphate, lead nitrate and organometallic lead compounds or compounds that can form lead oxides or salts during thermal decomposition, or any combinations thereof. In another embodiment, the glass frit may be lead- free. The term“lead-free” indicates that the glass frit has less than 0.5 wt% lead, based upon 100% total weight of the glass frit. The lead-free glass frit may include other oxides or compounds known to one skilled in the art, including, but not limited to, silicon, boron, aluminum, bismuth, lithium, sodium, magnesium, zinc, titanium, zirconium oxides, or compounds thereof.

[0045] In addition to the components recited above, the glass frit may also comprise other compounds used to improve the contact properties of the resulting electroconductive paste. For example, the glass frit may also comprise oxides or other compounds of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, V, Zr, Mo, Mn, Zn, B, P, Sn, Ga, Ge, In, Al, Sb, Bi, Ce, Cu, Ni, Cr, Fe, Co, or any combinations thereof. Examples of such oxides and compounds include, but are not limited to, germanium oxides, vanadium oxides, molybdenum oxides, niobium oxides, lithium oxides, tin oxides, indium oxides, rare earth oxides (such as La₂O₃ or cerium oxides), phosphorus oxides, transition metal oxides (such as copper oxides and chromium oxides), metal halides (such as lead fluorides and zinc fluorides), and combinations thereof. Such oxides and compounds are preferably present in an amount of at least about 0.1 wt%, and no more than about 15 wt%, based upon 100% total weight of the glass frit.

[0046] It is well known to the person skilled in the art that glass frit particles can exhibit a variety of shapes, sizes, and surface area to volume ratios. The glass particles may exhibit the same or similar shapes (including length:width:thickness ratio) as may be exhibited by the conductive metallic particles, as discussed herein. Glass frit particles with a shape, or combination of shapes, which favor improved electrical contact of the produced electrode are preferred.

[0047] It is preferred that the median particle diameter d₅₀ of the glass frit particles be at least about 0.1 μm. At the same time, it is preferred that the d₅₀ of the glass frit be no more than about 10 μm, more preferably no more than about 5 μm, and most preferably no more than about 3.5 μm. In one embodiment, the glass frit particles have a specific surface area of at least about 0.5 m²/g, preferably at least about 1 m²/g, and most preferably at least about 2 m²/g. At the same time, it is preferred that the specific surface area be no more than about 11 m²/g, preferably no more than about 10 m²/g, and most preferably no more than about 8 m²/g.

[0048] According to another embodiment, the glass frit particles may include a surface coating. Any such coating known in the art and which is considered to be suitable in the context of the invention can be employed on the glass frit particles. Preferred coatings according to the invention include those coatings which promote dispersion of the glass in the organic vehicle and improved contact of the electroconductive paste. If such a coating is present, it is preferred that the coating correspond to no more than about 10 wt%, preferably no more than about 8 wt%, most preferably no more than about 5 wt%, in each case based on the total weight of the glass frit particles. Additives

[0049] Preferred additives in the context of the invention are components added to the paste, in addition to the other components explicitly mentioned, which contribute to increased electrical performance of the paste, of the electrodes produced thereof, or of the resulting solar cell. In addition to additives present in the glass frit and in the vehicle, additives can also be present in the electroconductive paste. Preferred additives include, but are not limited to, thixotropic agents, viscosity regulators, emulsifiers, stabilizing agents or pH regulators, inorganic additives, thickeners and dispersants, or a combination of at least two thereof. Preferred inorganic organometallic additives include, but are not limited to, Mg, Ni, Te, W, Zn, Mg, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Rh, V, Y, Sb, P, Cu and Cr or a combination of at least two thereof, preferably Zn, Sb, Mn, Ni, W, Te, Rh, V, Y, Sb, P and Ru, or a combination of at least two thereof, oxides thereof, compounds which can generate those metal oxides on firing, or a mixture of at least two of the aforementioned metals, a mixture of at least two of the aforementioned oxides, a mixture of at least two of the aforementioned compounds which can generate those metal oxides on firing, or mixtures of two or more of any of the above mentioned.

[0050] According to one embodiment, the paste may include at least about 0.1 wt% additive(s). At the same time, the paste preferably includes no more than about 10 wt% additive(s), preferably no more than about 5 wt%, and most preferably no more than about 2 wt%, based upon 100% total weight of the paste.

[0051] In one embodiment, the electroconductive paste is free or substantially free of pigments, such as black pigments. The term“substantially free” as used herein means that the paste has less than 0.1 wt% pigment, based upon 100% total weight of the paste. Forming the Electroconductive Paste Composition

[0052] To form an electroconductive paste, the glass frit materials are combined with the conductive metallic particles and organic vehicle using any method known in the art for preparing a paste composition. The method of preparation is not critical, as long as it results in a homogenously dispersed paste. The components can be mixed, such as with a mixer, then passed through a three roll mill, for example, to make a dispersed uniform paste. In addition to mixing all of the components together simultaneously, the raw glass frit materials can be co- milled with silver particles, for example, in a ball mill for 2-24 hours to achieve a homogenous mixture of glass frit and silver particles, which are then combined with the organic solvent in a mixer. Solar Cells

[0053] The invention also relates to a solar cell. In one embodiment, the solar cell comprises a semiconductor substrate (e.g., a silicon wafer) and an electroconductive paste composition according to any of the embodiments described herein.

[0054] In another aspect, the invention relates to a solar cell prepared by a process which includes applying an electroconductive paste composition according to any of the embodiments described herein to a semiconductor substrate (e.g., a silicon wafer) and firing the semiconductor substrate. Silicon Wafer

[0055] Preferred wafers according to the invention have regions, among other regions of the solar cell, capable of absorbing light with high efficiency to yield electron-hole pairs and separating holes and electrons across a boundary with high efficiency, preferably across a p-n junction boundary. Preferred wafers according to the invention are those comprising a single body made up of a front doped layer and a back doped layer.

[0056] Preferably, the wafer comprises appropriately doped tetravalent elements, binary compounds, tertiary compounds or alloys. Preferred tetravalent elements in this context include, but are not limited to, silicon, germanium, or tin, preferably silicon. Preferred binary compounds include, but are not limited to, combinations of two or more tetravalent elements, binary compounds of a group III element with a group V element, binary com-pounds of a group II element with a group VI element or binary compounds of a group IV element with a group VI element. Preferred combinations of tetravalent elements include, but are not limited to, combinations of two or more elements selected from silicon, germanium, tin or carbon, preferably SiC. The preferred binary compounds of a group III element with a group V element is GaAs. According to a preferred embodiment of the invention, the wafer is silicon. The foregoing description, in which silicon is explicitly mentioned, also applies to other wafer compositions described herein.

[0057] The p-n junction boundary is located where the front doped layer and back doped layer of the wafer meet. In an n-type solar cell, the back doped layer is doped with an electron donating n-type dopant and the front doped layer is doped with an electron accepting or hole donating p-type dopant. In a p-type solar cell, the back doped layer is doped with p-type dopant and the front doped layer is doped with n-type dopant. According to a preferred embodiment of the invention, a wafer with a p-n junction boundary is prepared by first providing a doped silicon substrate and then applying a doped layer of the opposite type to one face of that substrate.

[0058] The doped silicon substrate can be prepared by any method known in the art and considered suitable for the invention. Preferred sources of silicon substrates according to the invention include, but are not limited to, mono-crystalline silicon, multi-crystalline silicon, amorphous silicon and upgraded metallurgical silicon, most preferably mono-crystalline silicon or multi-crystalline silicon. Doping to form the doped silicon substrate can be carried out simultaneously by adding the dopant during the preparation of the silicon substrate, or it can be carried out in a subsequent step. Doping subsequent to the preparation of the silicon substrate can be carried out by gas diffusion epitaxy, for example. Doped silicon substrates are also readily commercially available. According to one embodiment, the initial doping of the silicon substrate may be carried out simultaneously to its formation by adding dopant to the silicon mix. According to another embodiment, the application of the front doped layer and the highly doped back layer, if present, may be carried out by gas-phase epitaxy. This gas phase epitaxy is preferably carried out at a temperature of at least about 500 °C, preferably at least about 600^oC, and most preferably at least about 650^oC. At the same time, the temperature is preferably no more than about 900°C, preferably no more than about 800°C, and most preferably no more than about 750°C. The gas phase epitaxy is preferably carried out at a pressure of at least about 2 kPa, preferably at least about 10 kPa, and most preferably at least about 40 kPa. At the same, the pressure is preferably no more than about 100 kPa, preferably no more than about 80 kPa, and most preferably no more than about 70 kPa.

[0059] It is known in the art that silicon substrates can exhibit a number of shapes, surface textures and sizes. The shape of the substrate may include cuboid, disc, wafer and irregular polyhedron, to name a few. According to a preferred embodiment of the invention, the wafer is a cuboid with two dimensions which are similar, preferably equal, and a third dimension which is significantly smaller than the other two dimensions. The third dimension may be at least 100 times smaller than the first two dimensions. Further, silicon substrates with rough surfaces are preferred. One way to assess the roughness of the substrate is to evaluate the surface roughness parameter for a sub-surface of the substrate, which is small in comparison to the total surface area of the substrate, preferably less than about one hundredth of the total surface area, and which is essentially planar. The value of the surface roughness parameter is given by the ratio of the area of the sub-surface to the area of a theoretical surface formed by projecting that sub- surface onto the flat plane best fitted to the sub-surface by minimizing mean square displacement. A higher value of the surface roughness parameter indicates a rougher, more irregular surface and a lower value of the surface roughness parameter indicates a smoother, more even surface. According to the invention, the surface roughness of the silicon substrate is preferably modified so as to produce an optimum balance between a number of factors including, but not limited to, light absorption and adhesion to the surface.

[0060] The two larger dimensions of the silicon substrate can be varied to suit the application required of the resultant solar cell. It is preferred according to the invention for the thickness of the silicon wafer to be below about 0.5 mm, more preferably below about 0.3 mm, and most preferably below about 0.2 mm. Some wafers have a minimum thickness of 0.01 mm or more.

[0061] It is preferred that the front doped layer be thin in comparison to the back doped layer. It is also preferred that the front doped layer have a thickness of at least about 0.1 μm, and preferably no more than about 10 μm, preferably no more than about 5 m, and most preferably no more than about 2 μm.

[0062] A highly doped layer can be applied to the back face of the silicon substrate between the back doped layer and any further layers. Such a highly doped layer is of the same doping type as the back doped layer and such a layer is commonly denoted with a + (n+-type layers are applied to n-type back doped layers and p+-type layers are applied to p-type back doped layers). This highly doped back layer serves to assist metallization and improve electroconductive properties. It is preferred according to the invention for the highly doped back layer, if present, to have a thickness of at least 1 μm, and preferably no more than about 100 μm, preferably no more than about 50 m and most preferably no more than about 15 μm. Dopants

[0063] Preferred dopants are those which, when added to the silicon wafer, form a p-n junction boundary by introducing electrons or holes into the band structure. It is preferred that the identity and concentration of these dopants is specifically selected so as to tune the band structure profile of the p-n junction and set the light absorption and conductivity profiles as required. Preferred p-type dopants include, but are not limited to, those which add holes to the silicon wafer band structure. All dopants known in the art and which are considered suitable in the context of the invention can be employed as p-type dopants. Preferred p-type dopants include, but are not limited to, trivalent elements, particularly those of group 13 of the periodic table. Preferred group 13 elements of the periodic table in this context include, but are not limited to, boron, aluminum, gallium, indium, thallium, or a combination of at least two thereof, wherein boron is particularly preferred.

[0064] Preferred n-type dopants are those which add electrons to the silicon wafer band structure. Preferred n-type dopants are elements of group 15 of the periodic table. Preferred group 15 elements of the periodic table in this context include, but are not limited to, nitrogen, phosphorus, arsenic, antimony, bismuth or a combination of at least two thereof, wherein phosphorus is particularly preferred.

[0065] As described above, the various doping levels of the p-n junction can be varied so as to tune the desired properties of the resulting solar cell. Doping levels are measured using secondary ion mass spectroscopy.

[0066] According to certain embodiments, the semiconductor substrate (i.e., silicon wafer) exhibits a sheet resistance above about 60 / , such as above about 65 / , 70 / , 90 / or 100 / . For measuring the sheet resistance of a doped silicon wafer surface, the device“GP4- Test Pro” equipped with software package“GP-4 Test 1.6.6 Pro” (available from GP Solar GmbH) is used. For the measurement, the four point measuring principle is applied. The two outer probes apply a constant current and two inner probes measure the voltage. The sheet resistance is deduced using the Ohmic law in / . To determine the average sheet resistance, the measurement is performed on 25 equally distributed spots of the wafer. In an air conditioned room with a temperature of 22 ± 1 °C, all equipment and materials are equilibrated before the measurement. To perform the measurement, the“GP-Test.Pro” is equipped with a 4-point measuring head (Part Number 04.01.0018) with sharp tips in order to penetrate the anti-reflection and/or passivation layers. A current of 10 mA is applied. The measuring head is brought into contact with the non metalized wafer material and the measurement is started. After measuring 25 equally distributed spots on the wafer, the average sheet resistance is calculated in / . Solar Cell Structure

[0067] A contribution to achieving at least one of the above described objects is made by a solar cell obtainable from a process according to the invention. Preferred solar cells according to the invention are those which have a high efficiency, in terms of proportion of total energy of incident light converted into electrical energy output, and those which are light and durable. At a minimum, a solar cell includes: (i) front electrodes, (ii) a front doped layer, (iii) a p-n junction boundary, (iv) a back doped layer, and (v) soldering pads. The solar cell may also include additional layers for chemical/mechanical protection. Antireflective Layer

[0068] According to the invention, an antireflective layer may be applied as the outer layer before the electrode is applied to the front face of the solar cell. All antireflective layers known in the art and which are considered to be suitable in the context of the invention can be employed. Preferred antireflective layers are those which decrease the proportion of incident light reflected by the front face and increase the proportion of incident light crossing the front face to be absorbed by the wafer. Antireflective layers which give rise to a favorable absorption/reflection ratio, are susceptible to etching by the electroconductive paste, are otherwise resistant to the temperatures required for firing of the electroconductive paste, and do not contribute to increased recombination of electrons and holes in the vicinity of the electrode interface, are preferred. Preferred antireflective layers include, but are not limited to, SiN_x, SiO₂, Al₂O₃, TiO₂ or mixtures of at least two thereof and/or combinations of at least two layers thereof. According to a preferred embodiment, the antireflective layer is SiN_x, in particular where a silicon wafer is employed.

[0069] The thickness of antireflective layers is suited to the wavelength of the appropriate light. According to a preferred embodiment of the invention, the antireflective layers have a thickness of at least 20 nm, preferably at least 40 nm, and most preferably at least 60 nm. At the same time, the thickness is preferably no more than about 300 nm, more preferably no more than about 200 nm, and most preferably no more than about 90 nm. Passivation Layers

[0070] One or more passivation layers may be applied to the front and/or back side of the silicon wafer as an outer layer. The passivation layer(s) may be applied before the front electrode is formed, or before the antireflective layer is applied (if one is present). Preferred passivation layers are those which reduce the rate of electron/hole recombination in the vicinity of the electrode interface. Any passivation layer which is known in the art and which is considered to be suitable in the context of the invention can be employed. Preferred passivation layers according to the invention include, but are not limited to, silicon nitride, silicon dioxide and titanium dioxide. According to a more preferred embodiment, silicon nitride is used. It is preferred for the passivation layer to have a thickness of at least 0.1 nm, preferably at least 10 nm, and most preferably at least 30 nm. As the same time, the thickness is preferably no more than about 2 μm, preferably no more than about 1 μm, and most preferably no more than about 200 nm. Additional Protective Layers

[0071] In addition to the layers described above, further layers can be added for mechanical and chemical protection. The cell can be encapsulated to provide chemical protection. According to a preferred embodiment, transparent polymers, often referred to as transparent thermoplastic resins, are used as the encapsulation material, if such an encapsulation is present. Preferred transparent polymers in this context are silicon rubber and polyethylene vinyl acetate (PVA). A transparent glass sheet may also be added to the front of the solar cell to provide mechanical protection to the front face of the cell. A back protecting material may be added to the back face of the solar cell to provide mechanical protection. Preferred back protecting materials are those having good mechanical properties and weather resistance. The preferred back protection material according to the invention is polyethylene terephthalate with a layer of polyvinyl fluoride. It is preferred for the back protecting material to be present underneath the encapsulation layer (in the event that both a back protection layer and encapsulation are present).

[0072] A frame material can be added to the outside of the solar cell to give mechanical support. Frame materials are well known in the art and any frame material considered suitable in the context of the invention may be employed. The preferred frame material according to the invention is aluminum. Method of Preparing a Solar Cell

[0073] A solar cell may be prepared by applying the electroconductive paste of the invention to an antireflection coating, such as silicon nitride, silicon oxide, titanium oxide or aluminum oxide, on the front side of a semiconductor substrate, such as a silicon wafer. A backside electroconductive paste is then applied to the backside of the solar cell to form soldering pads. An aluminum paste is then applied to the backside of the substrate, overlapping the edges of the soldering pads formed from the backside electroconductive paste, to form the BSF.

[0074] The electroconductive pastes may be applied in any manner known in the art and considered suitable in the context of the invention. Examples include, but are not limited to, impregnation, dipping, pouring, dripping on, injection, spraying, knife coating, curtain coating, brushing or printing or a combination of at least two thereof. Preferred printing techniques are ink-jet printing, screen printing, tampon printing, offset printing, relief printing or stencil printing or a combination of at least two thereof. It is preferred according to the invention that the electroconductive paste is applied by printing, preferably by screen printing. Specifically, the screens preferably have mesh opening with a diameter of about 40 μm or less (e.g., about 35 μm or less, about 30 μm or less). At the same time, the screens preferably have a mesh opening with a diameter of at least 10 μm.

[0075] The substrate is then fired according to an appropriate profile. Firing is necessary to sinter the printed electroconductive paste so as to form solid electrodes. Firing is well known in the art and can be effected in any manner considered suitable in the context of the invention. It is preferred that firing be carried out above the T_g of the glass frit materials.

[0076] According to the invention, the maximum temperature set for firing is below about 900°C, preferably below about 860°C. Firing temperatures as low as about 800°C have been employed for obtaining solar cells. Firing temperatures should also allow for effective sintering of the metallic particles to be achieved. The firing temperature profile is typically set so as to enable the burnout of organic materials from the electroconductive paste composition. The firing step is typically carried out in air or in an oxygen-containing atmosphere in a belt furnace. It is preferred for firing to be carried out in a fast firing process with a total firing time of at least 30 seconds, and preferably at least 40 seconds. At the same time, the firing time is preferably no more than about 3 minutes, more preferably no more than about 2 minutes, and most preferably no more than about 1 minute. The time above 600°C is most preferably in a range from about 3 to 7 seconds. The substrate may reach a peak temperature in the range of about 700 to 900ºC for a period of about 1 to 5 seconds. The firing may also be conducted at high transport rates, for example, about 100-700 cm/min, with resulting hold-up times of about 0.5 to 3 minutes. Multiple temperature zones, for example 3-12 zones, can be used to control the desired thermal profile.

[0077] Firing of electroconductive pastes on the front and back faces can be carried out simultaneously or sequentially. Simultaneous firing is appropriate if the electroconductive pastes applied to both faces have similar, preferably identical, optimum firing conditions. Where appropriate, it is preferred for firing to be carried out simultaneously. Where firing is carried out sequentially, it is preferable for the back electroconductive paste to be applied and fired first, followed by application and firing of the electroconductive paste to the front face of the substrate. Measuring Properties of Electroconductive Paste

[0078] The sample solar cell is characterized using a commercial IV-tester“cetisPV-CTL1” from Halm Elektronik GmbH. All parts of the measurement equipment as well as the solar cell to be tested were maintained at 25°C during electrical measurement. This temperature should be measured simultaneously on the cell surface during the actual measurement by a temperature probe. The Xe Arc lamp simulates the sunlight with a known AM1.5 intensity of 1000 W/m² on the cell surface. To bring the simulator to this intensity, the lamp is flashed several times within a short period of time until it reaches a stable level monitored by the“PVCTControl 4.313.0” software of the IV-tester. The Halm IV tester uses a multi-point contact method to measure current (I) and voltage (V) to determine the solar cell’s IV-curve. To do so, the solar cell is placed between the multi-point contact probes in such a way that the probe fingers are in contact with the bus bars (i.e., printed lines) of the solar cell. The numbers of contact probe lines are adjusted to the number of bus bars on the cell surface. All electrical values were determined directly from this curve automatically by the implemented software package. As a reference standard, a calibrated solar cell from ISE Freiburg consisting of the same area dimensions, same wafer material, and processed using the same front side layout, was tested and the data was compared to the certificated values. At least five wafers processed in the very same way were measured and the data was interpreted by calculating the average of each value. The software PVCTControl 4.313.0 provided values for efficiency, fill factor, short circuit current, series resistance and open circuit voltage.

[0079] To measure viscosity of the electroconductive paste, a Brookfield HBDV-III Digital Rheometer equipped with a CP-44Y sample cup and a #51 cone was used. The temperature of the sample was maintained at 25^oC using a TC-502 circulating temperature bath. The measurement gap was set at 0.026 mm with a sample volume of approximately 0.5 ml. The sample was allowed to equilibrate for two minutes, and then a constant rotational speed of 1.0 rpm was applied for one minute. The viscosity of the sample after this interval was reported in units of kcps. Solar Cell Module

[0080] A plurality of solar cells according to the invention can be arranged spatially and electrically connected to form a collective arrangement called a module. Preferred modules according to the invention can have a number of arrangements, preferably a rectangular arrangement known as a solar panel. A large variety of ways to electrically connect solar cells, as well as a large variety of ways to mechanically arrange and fix such cells to form collective arrangements, are well known in the art. Preferred methods according to the invention are those which result in a low mass to power output ratio, low volume to power output ration, and high durability. Aluminum is the preferred material for mechanical fixing of solar cells according to the invention.

[0081] In one embodiment, multiple solar cells are connected in series and/or in parallel and the ends of the electrodes of the first cell and the last cell are preferably connected to output wiring. The solar cells are typically encapsulated in a transparent thermal plastic resin, such as silicon rubber or ethylene vinyl acetate. A transparent sheet of glass is placed on the front surface of the encapsulating transparent thermal plastic resin. A back protecting material, for example, a sheet of polyethylene terephthalate coated with a film of polyvinyl fluoride, is placed under the encapsulating thermal plastic resin. These layered materials may be heated in an appropriate vacuum furnace to remove air, and then integrated into one body by heating and pressing. Furthermore, since solar cells are typically left in the open air for a long time, it is desirable to cover the circumference of the solar cell with a frame material consisting of aluminum or the like.

[0082] The invention will now be described in conjunction with the following, non-limiting example. Example

[0083] An exemplary organic vehicle was prepared by combining about 33 wt% texanol, about 33 wt% of a diethylene glycol monobutyl ether solvent, about 16.5 wt% of a diethylene glycol monobutyl ether acetate solvent, about 0.5 wt% of a surfactant, about 12 wt% of a thixotropic agent, and about 5 wt% of an acrylic resin (Elvacite^® 2045 manufactured by Lucite International, Inc.). All weight percentages are based upon 100% total weight of the organic vehicle. The mixture was heated to a temperature of 65^oC while stirring, and was then maintained for a total of 30 minutes at that temperature. The vehicle was then cooled and milled using a three-roll mill until it reached a homogeneous consistency.

[0084] An exemplary electroconductive paste was then prepared by mixing about 9 wt% of the organic vehicle described above, about 87.5 wt% silver particles, about 3.5 wt% glass frit, and about 0.2 wt% additional surfactant. The mixture was then milled using a three-roll mill until it became a dispersed uniform paste.

[0085] The viscosity and solids loading for the exemplary paste, as well as a control paste (not having acrylic resin), were then measured according to the parameters set forth herein. The solids loading is the amount of solids (i.e., glass and conductive particles) in the paste composition, based upon 100% total weight of the paste. The measurements are set forth in Table 1 below.

[0086] The exemplary paste and control paste were then screen printed onto a silicon wafer at a speed of 150 mm/s, using screen 325 (mesh) * 0.9 (mil, wire diameter) * 0.6 (mil, emulsion thickness) * 40 μm (finger line opening) (Calendar screen). The amount of paste deposited onto the silicon wafer (before drying and firing) was also measured and is set forth in Table 1. The printed wafers were then dried at about 150^oC and fired at a profile with a peak temperature at about 800^oC for a few seconds in a linear multi-zone infrared furnace. Table 1. Printing Parameters for Exemplary Solar Cell

[0087] The printed finger lines were then photographed and measured for analysis. As set forth in Table 2 below, the height and width of each finger line were measured along their length, and the mean, median, minimum, and maximum value for each measurement was calculated. The lines were measured using a Zeta-200 Optical Profiler, manufactured by Zeta Instruments of San Jose, California. The aspect ratio was also calculated by dividing the mean height by the mean width. Table 2. Printed Finger Line Characteristics

[0088] The exemplary pastes exhibited a higher aspect ratio, as evidenced by the higher mean height and lower mean width.

[0089] The resulting solar cells were then tested using an I-V tester according to the parameters set forth herein. Electrical performance characteristics including Eta (efficiency), short circuit current density (Isc), open circuit voltage (Voc), Fill Factor (FF), series resistance under three standard lighting intensities (Rs3), and shunt resistance (Rsh) of both the control paste and the exemplary paste are provided in Table 3 below. All data for the control and exemplary pastes was normalized to 1. The normalized data for the experimental paste was calculated by dividing the relevant exemplary measurement by the normalized control cell measurement. Table 3. Electrical Performance of Exemplary Solar Cell

[0090] The finger lines printed with the exemplary paste demonstrated improved electrical performance over lines printed with the control paste. The exemplary paste exhibited increased efficiency, short circuit current density, and open circuit voltage as compared to the control paste. It is believed that the larger mean height and decreased mean width of the exemplary line (higher aspect ratio) contributed to its improved electrical performance.

[0091] These and other advantages of the invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it will be recognized by those skilled in the art that changes or modifications may be made to the above described embodiments without departing from the broad inventive concepts of the invention. Specific dimensions of any particular embodiment are described for illustration purposes only. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention.

Claims

What is claimed: 1. An electroconductive paste composition for manufacturing a solar cell comprising:

(a) conductive metallic particles;

(b) glass frit; and

(c) an organic vehicle comprising

(i) an organic solvent, and

(ii) an acrylic resin lacking an active hydrogen functionality.

2. The electroconductive paste composition according to claim 1, wherein the conductive metallic particles are at least 35 wt%, preferably at least 50 wt%, more preferably at least 70 wt%, and most preferably at least 80 wt%, and no more than about 99 wt%, preferably no more than about 95 wt%, based upon 100% total weight of the paste.

3. The electroconductive paste composition according to claims 1 or 2, wherein the conductive metallic particles are at least one of silver, gold, copper, aluminum, nickel, and mixtures or alloys thereof, preferably silver.

4. The electroconductive paste composition according to any of the preceding claims, wherein the glass frit is at least about 0.5 wt%, preferably at least about 1 wt%, and no more than about 15 wt%, preferably no more than about 10 wt%, and most preferably no more than about 6 wt%, based upon 100% total weight of the paste.

5. The electroconductive paste composition according to any of the preceding claims, wherein the organic vehicle is at least about 1 wt%, more preferably at least about 5 wt%, and no more than about 20 wt%, preferably no more than about 15 wt%, based upon 100% total weight of the paste.

6. The electroconductive paste composition according to any of the preceding claims, wherein the organic vehicle includes at least about 1 wt% acrylic resin, preferably at least about 2 wt%, and most preferably at least about 4 wt%, and no more than about 15 wt% acrylic resin, preferably no more than about 12 wt%, and most preferably no more than about 10 wt%, based upon 100% total weight of the organic vehicle.

7. The electroconductive paste composition according to any of the preceding claims, wherein the acrylic resin is an isobutyl methacrylate resin.

8. The electroconductive paste composition according to any of the preceding claims, wherein the acrylic resin has a molecular weight of at least about 50 KD, and preferably at least about 100 KD, and no more than about 350 KD, preferably no more than about 300 KD, and most preferably no more than about 250 KD.

9. The electroconductive paste composition according to any of the preceding claims, wherein the organic vehicle further comprises a thixotropic agent.

10. The electroconductive paste composition according to claim 9, wherein the organic vehicle comprises at least about 5 wt% thixotropic agent, and preferably at least about 8 wt%, and no more than about 15 wt%, and preferably no more than about 13 wt%, based upon 100% total weight of the organic vehicle.

11. The electroconductive paste composition according to any of the preceding claims, wherein the organic vehicle further comprises at least about 0.01 wt% surfactant, and no more than about 10 wt% surfactant, preferably no more than about 5 wt%, and most preferably no more than about 3 wt%, based upon 100% total weight of the organic vehicle.

12. The electroconductive paste composition according to any of the preceding claims, wherein the organic vehicle comprises at least about 60 wt% organic solvent, preferably at least about 70 wt%, and most preferably at least about 80 wt%, and no more than about 90 wt% solvent, based upon 100% total weight of the paste.

13. The electroconductive paste composition according to any of the preceding claims, wherein the organic solvent comprises texanol, diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, or any combination thereof.

14. The electroconductive paste composition according to any of the preceding claims, wherein the organic vehicle does not include a urethane acrylate resin.

15. The electroconductive paste composition according to any of the preceding claims, wherein the paste composition has a viscosity of at least about 50 Kcps and at most about 300 Kcps.

16. The electroconductive paste composition according to any of the preceding claims, wherein the paste composition is free or substantially free of pigments.

17. The electroconductive paste composition according to any of the preceding claims, wherein the paste composition is capable of being screen printed through screen openings having a diameter of about 40 μm or less.

18. A solar cell produced by applying an electroconductive paste according to claims 1-17 to a silicon wafer and firing the silicon wafer.

19. A method of producing a solar cell, comprising the steps of:

providing a silicon wafer having a front side and a backside; applying an electroconductive paste according to any of claims 1-17 to the silicon wafer; and

firing the silicon wafer.

20. The method of producing a solar cell according to claim 19, wherein the

electroconductive paste is applied to the front side of the silicon wafer.