TW201529655A - Acrylic resin-containing organic vehicle for electroconductive paste - Google Patents

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 [mu]m or less.

Description

Organic carrier containing acrylic resin for conductive paste

The present invention relates to a conductive paste containing an organic vehicle, which comprises an acrylic resin. These pastes can be used to make solar cells.

Solar cells use photovoltaics to convert light energy into electrical devices. Solar energy is an attractive green energy source because it is sustainable and produces only pollution-free by-products. In operation, when light strikes the solar cell, one portion of the incident light is partially reflected by the surface and the remainder is transmitted into the solar cell. Photons of transmitted light are absorbed by solar cells, which are typically made of a semiconducting material such as germanium. The energy from the absorbed photons excites electrons from the atoms of the semiconducting material, thereby generating electron-hole pairs. The electron-hole pairs are then separated by a p-n junction and collected by a conductive electrode applied to the surface of the solar cell. In this way, electricity can be conducted between interconnected solar cells.

Solar cells typically have a conductive paste that is applied to the front and back surfaces of the solar cell and forms an electrode upon firing. The front side paste is typically screen printed onto the front side of the substrate to serve as a front electrode. Typical conductive pastes contain metal particles, glass frits, and organic carriers. These components should be carefully selected to fully utilize the potential of the resulting solar cell. For example, the composition of the organic vehicle can have an impact on the performance of the resulting solar array. The organic vehicle has an effect on the viscosity of the paste, thereby affecting its printability. One problem associated with known front side paste compositions is that they are not readily permeable via having less than about 40 [mu]m. Screen printing of the diameter of the opening. When the paste is not easy to print, the resulting printed lines forming the electrodes may be uneven, thus affecting the overall efficiency of the resulting solar cells.

Chinese Patent Publication No. 101834008 discloses self-crosslinking acrylate resins for use in electronic silver pastes. The paste contains molecules having functional groups (for example, acrylates, epoxides) that are slowly crosslinked at room temperature, thereby increasing the viscosity of the paste and reducing storage stability. The resin is not suitable for use in higher temperature application pastes because of the accelerated cross-linking of heat which increases the organic molecular weight and inhibits the cleanliness of the organic phase.

Japanese Patent Application Publication No. 2009/259826 discloses a black conductive paste for application to a plasma display panel. This paste includes acrylate polymer resins and also includes black pigments that are not suitable for forming solar cell electrodes.

Accordingly, there is a need for an improved conductive paste having a viscosity suitable for printing through a screen opening having a screen opening having a diameter of about 40 [mu]m or less and suitable for a solar cell.

One embodiment of the present invention relates to a conductive paste composition for use in the manufacture of solar cells. The paste contains conductive metal particles, a glass frit, and an organic vehicle, and the organic vehicle includes (i) an organic solvent and (ii) an acrylic resin having no active hydrogen functional group.

The conductive paste of the present invention is suitable for printing through a screen opening having a diameter of about 40 μm or less, thereby allowing fine line printing while still providing sufficient conductivity.

Another embodiment of the present invention is directed to a method of producing a solar cell comprising the steps of: providing a tantalum wafer having a front side and a back side, applying a conductive paste of the present invention to a tantalum wafer, and firing a tantalum wafer.

The paste of the present invention can be used to form electrodes on solar cells. The electrodes provide a means of conducting electrical conductivity between the solar cells. The conductive paste composition preferably includes electrical conductivity Metal particles, glass frits, and organic carriers including acrylic resins.

Organic carrier

The organic carrier is expected to allow fine line printability while also improving the physical properties of the printed line. In particular, conductive pastes that can be screen printed and form a uniform (or substantially uniform) printed line are preferred. One way to characterize the uniformity of a printed line is by its line definition, which can be analyzed by analyzing the longitudinal edge uniformity (ie, the width along the length of the line) and the height change (ie, the "peak" along the top of the line and "Valley") to measure. The standard deviation of the line edges or height is typically used to determine line uniformity. The lower the standard deviation, the better the line uniformity considering the line width and line height values.

This uniformity is impaired when the current paste is screen printed via a screen opening having a diameter of about 40 μm or less. With these narrow screen openings, the viscosity of the paste should be reduced (compared to the paste printed via the larger screen opening) to facilitate the deposition of the paste through the screen. As the viscosity decreases, the line definition deteriorates, and the paste is more likely to "expand" along the surface of the substrate. Similarly, the aspect ratio (the ratio between height to width) is also reduced. With reduced line uniformity and reduced aspect ratio, the efficiency of solar cells deteriorates. Generally, a cross-section of more than about 0.25 is preferred.

Beneficially, a relatively small screen opening (i.e., 40 [mu]m or less) is used to print to create a thinner line, thereby less covering the wafer surface. By covering the surface with less conductive paste, more of the surface is exposed to sunlight, which increases the efficiency of the solar cell. The present invention allows for the printing of fine lines while maintaining line uniformity and aspect ratio.

In a preferred embodiment, the paste has a viscosity of at least about 50 Kcp and up to about 300 Kcp (thousand centipoise). Without being bound by any particular theory, it is believed that this viscosity range allows for an optimum screen to be printed via a screen opening having a diameter of about 40 [mu]m or less.

In one embodiment, the electrically conductive paste comprises at least about 1 wt% organic vehicle and more preferably at least about 5 wt% organic vehicle, based on 100% total weight of the paste. Meanwhile, the conductive paste preferably includes no more than about 20% by weight of the organic vehicle and more based on 100% by weight of the paste. Preferably no more than about 15% by weight of the organic vehicle.

According to a preferred embodiment, the organic vehicle comprises an acrylic resin. Current conductive paste compositions often use cellulose or cellulose esters as the resin component of the organic vehicle. However, the pastes cannot be sufficiently printed via screen openings having a diameter of about 40 [mu]m or less. The use of an acrylic resin instead of cellulose or cellulose ester or the use of an acrylic resin in addition to cellulose or cellulose ester improves the printability of a screen using a diameter of 40 μm or less, so that the uniformity of the printed line is improved.

The organic vehicle may comprise at least about 1% by weight, preferably at least about 2% by weight and most preferably at least about 4% by weight, based on 100% by weight of the organic vehicle. Also, the organic vehicle preferably comprises no more than about 15% by weight, preferably no more than 12% and most preferably no more than 10% by weight based on 100% by weight of the organic vehicle.

The acrylic resin preferably has no active hydrogen functional groups (i.e., hydrogen atoms that can be supplied, received or shared). For example, the acrylic resin preferably has no hydroxyl group, carboxyl group or guanamine group (except for the carboxyl group in the center of the acrylate). In one embodiment, the acrylic resin may be a polymer of an alkyl methacrylate such as a C ₁ -C ₆ methacrylate. The alkyl group can be straight 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 can be any other acrylate resin having no active hydrogen functional groups including, but not limited to, butyl acrylate or isobutyl acrylate. According to one embodiment, the acrylic resin is not self-crosslinking.

The acrylic resin preferably has a molecular weight of at least about 50 kilodaltons (KD) and preferably at least about 100 kD. Also, the acrylic resin preferably has a molecular weight of not more than about 350 KD, preferably not more than about 300 KD, and most preferably not more than about 250 KD. Preferred acrylic resins are, for example, isobutyl methacrylate polymers having a molecular weight of from about 100 to 250 KD (e.g., 193 KD).

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

According to another embodiment, the organic vehicle further comprises 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.

The solvent component is preferably removed from the paste to a large extent during firing. In one embodiment, a solvent present after firing is preferably at least about 80%, more preferably at least 95%, of the absolute weight reduction prior to firing. Preferred solvents are those which provide improved viscosity, printability, stability and sintering characteristics to the paste. All solvents known in the art and considered suitable for the context of the present invention are useful as solvents in organic vehicles. Preferred solvents are those which exist in liquid form at standard ambient temperature and pressure (SATP) (298.15K, 25 ° C, 77 ° F), 100 kPa (14.504 psi, 0.986 atm), and more preferably have a higher than about The boiling point of 90 ° C and the melting point above about 20 ° C.

Preferred solvents for the present invention include, but are not limited to, polar or non-polar, protic or aprotic, aromatic or non-aromatic compounds, and may be monoalcohols, diols, polyols, monoesters, diesters, polyesters. a monoether, a diether, a polyether, at least one or more of the functional groups comprising the classes, and optionally other functional groups (preferably a cyclic group, an aromatic group, an unsaturated bond, An alcohol group in which one or more O atoms are replaced by a hetero atom (for example, an N atom), an ether group in which one or more O atoms are replaced by a hetero atom (for example, an N atom), and one or more O atoms from a hetero atom ( For example, a solvent of an N atom (alternative ester group) and a mixture of two or more of the above solvents. Preferred esters in this context include, but are not limited to, di-alkyl esters of fatty acids, preferably alkyl components including methyl, ethyl, propyl, butyl, pentyl, hexyl and higher alkyl Or a combination of two different such alkyl groups, preferably a mixture of dimethyl adipate and two or more adipates. 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 component of the ethyl dialkyl ether may be, for example, methyl, ethyl, propyl, butyl, pentyl, hexyl and higher alkyl or two different alkyls. combination. Preferred alcohols in this context include, but are not limited to, primary, secondary and tertiary alcohols, preferably tertiary alcohols, terpineol and its derivatives, or both. A mixture of more or more alcohols. Preferred solvents for combining more than one functional group include, but are not limited to, (i) 2,2,4-trimethyl-1,3-pentanediol isobutyrate (commonly known as texanol) and a derivative thereof, (ii) 2-(2-ethoxyethoxy)ethanol (also known as carbitol) and an alkyl derivative thereof, preferably methyl, ethyl, propyl, butyl, pentane And hexyl carbitol, preferably hexyl carbitol or butyl carbitol and its acetate derivative, preferably butyl carbitol acetate, or (iii) a mixture of at least two of the foregoing.

In one embodiment, the solvent comprises diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, or any combination thereof.

The organic vehicle may comprise at least about 60% by weight, preferably at least about 70% by weight and optimally at least about 80% by weight, based on the total weight of the organic vehicle. Also, the organic vehicle preferably comprises no more than about 90% by weight of an organic solvent based on 100% by weight of the organic vehicle.

According to another embodiment, the organic vehicle also includes a surfactant. Preferred surfactants in the context of the present invention are those which promote the formation of conductive pastes having advantageous stability, printability, viscosity and sintering properties. Preferred surfactants include, but are not limited to, those based on linear, branched, aromatic, fluorinated, decyl, polyether, and combinations thereof, and may be single-stranded , double or multiple chains. Preferred surfactants can have nonionic, 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 a pigment-affinity group, preferably a hydroxy-functional carboxylic acid ester having a pigment-affinity group (for example, DISPERBYK®-108, manufactured by BYK USA), an acrylate copolymer having a pigment affinity group. (for example, DISPERBYK®-116, manufactured by BYK USA), modified polyether with pigment affinity groups (for example, TEGO® DISPERS 655, manufactured by Evonik Tego Chemie GmbH) or other groups with high pigment affinity A surfactant (for example, TEGO® DISPERS 662 C, manufactured by Evonik Tego Chemie GmbH). Other preferred surfactants include, but are not limited to, polyethylene oxide, polyethylene glycol and derivatives thereof, and alkyl carboxylic acids and derivatives or salts thereof or mixtures thereof. A preferred polyethylene glycol derivative is poly(ethyl diol) acetic acid. Preferred alkyl carboxylic acids are those which are fully saturated and those which have a single or polyunsaturated alkyl chain or mixtures thereof. Preferred carboxylic acids having a saturated alkyl chain are those having an alkyl chain length in the range of from about 8 to about 20 carbon atoms, preferably C ₉ H ₁₉ COOH (capric acid), C ₁₁ H ₂₃ COOH (lauric acid). ), C ₁₃ H ₂₇ COOH (myristic acid), C ₁₅ H ₃₁ COOH (palmitic acid), C ₁₇ H ₃₅ COOH (stearic acid) or a salt or mixture thereof. Preferred carboxylic acids having an unsaturated alkyl chain are C ₁₈ H ₃₄ O ₂ (oleic acid) and C ₁₈ H ₃₂ O ₂ (linoleic acid). A preferred combination comprising an alkyl carboxylic acid in this context is castor oil. Preferred monomeric surfactants are benzotriazole and its derivatives.

The surfactant can be present in the organic vehicle in an amount of at least about 0.01% by weight. Also, the surfactant preferably does not exceed about 10% by weight, preferably does not exceed about 5% by weight and more preferably does not exceed about 3% by weight based on 100% by weight of the organic vehicle.

The organic vehicle may also contain a thixotropic agent. Any thixotropic agent known to those skilled in the art can be used with the organic vehicle of the present invention. By way of example and not limitation, the thixotropic agent can be derived from a natural source (eg, castor oil) or it can be synthesized. Preferred thixotropic agents are carboxylic acid derivatives, preferably fatty acid derivatives or combinations thereof. Commercially available thixotropic agents such as Thixotrol ^® MAX can also be used. According to a preferred embodiment, the organic vehicle comprises at least about 5% by weight and preferably at least about 8% by weight, based on 100% by weight of the organic vehicle. Also, the organic vehicle preferably comprises no more than about 15% by weight, preferably no more than about 13% by weight, based on 100% by weight of the organic vehicle.

The organic vehicle may also contain one or more additives. Preferred additives in the organic vehicle are those which are different from the above-mentioned carrier component and which contribute to the advantageous viscosity and printability of the conductive paste. Preferred additives include, but are not limited to, viscosity modifiers, stabilizers, inorganic additives, thickeners, emulsifiers, dispersants, or pH adjusters. Preferred surfactants are carboxylic acids having a saturated alkyl chain, such as those having an alkyl chain length in the range of from about 8 to about 20 carbon atoms, preferably C9H19COOH (decanoic acid), C11H23COOH (lauric acid), C13H27COOH. (Myristic acid), C15H31COOH (palmitic acid), C17H35COOH (stearic acid) or a salt or mixture thereof. Preferred carboxylic acids having an unsaturated alkyl chain are C18H34O2 (oleic acid) and C18H32O2 (linolenic acid).

Conductive metal particles

The conductive metal particles in the context of the present invention exhibit optimal conductivity and are effectively sintered at the time of firing such that they produce electrodes having high conductivity. Conductive metal particles suitable for forming solar cell electrodes are known in the art to be preferred. Preferred metal 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 and at least one alloy.

The conductive paste may comprise at least 35 wt%, preferably at least 50 wt%, more preferably at least 70 wt%, and most preferably at least 80 wt% metal particles, based on 100% by weight of the paste. Also, the conductive paste preferably comprises no more than about 99% by weight, preferably no more than about 95% by weight, based on the total weight of the paste. A conductive paste having a metal particle content of less than 35 wt% may not provide sufficient conductivity and adhesion, and a conductive paste having a metal particle content of more than 95 wt% may have an excessively high viscosity for a suitable screen printing.

Metals useful as metal particles include at least one of silver, copper, gold, aluminum, nickel, and any mixture or alloy of at least two thereof. In a preferred embodiment, the metal particles are silver. Silver can be formed by elemental silver, silver alloy or silver derivatives. Suitable silver derivatives include, for example, silver alloys and/or silver salts (eg, silver halides (eg, silver chloride), silver oxide, silver nitrate, silver acetate, silver trifluoroacetate, silver orthophosphate, and combinations thereof. In embodiments, the metal particles may comprise a metal or alloy coated with one or more different metals or alloys, such as aluminum coated silver particles or silver coated copper particles.

Metal particles may be coated with an organic or inorganic surface. Any such coating known in the art and considered suitable for the context of the present invention can be used on metal particles. Preferred organic coatings promote their dispersion into the organic vehicle. Preferred inorganic coatings are those which sinter and which promote the adhesion properties of the resulting conductive paste. If present, the coating preferably corresponds to no more than about 5% by weight, preferably no more than about 2% by weight, based on 100% by total weight of the metal particles. And preferably no more than about 1% by weight.

The electrically conductive particles can take on a variety of shapes, surfaces, sizes, and surface area to volume ratios. Some examples of shapes include, but are not limited to, spherical, angular, stretched (rod or needle), and planar (flaky). The conductive metal particles may also be present in a combination of particles having different shapes. One way to characterize these shapes is through the following parameters: length, width, and thickness. In the context of the present invention, the length of a particle is given by the length of the longest spatial displacement vector whose two endpoints are contained within the particle. The width of the particle is given by the length of the longest spatial displacement vector perpendicular to the length vector of the two endpoints defined above that are contained within the particle. The thickness of the particles is given by the length of the longest spatial displacement vector perpendicular to both the length vector and the width vector (both of which are defined above, with their two endpoints contained within the particle).

In one embodiment, it is preferred that the metal particles have a shape that is as uniform as possible (ie, the ratio of any of length, width, and thickness is as close as possible to 1, preferably at least 0.7, more preferably at least 0.8, and optimal. A shape of 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 of metal particles in this embodiment are spheres and cubes or combinations thereof or combinations of one or more thereof with other shapes. In another embodiment, the metal particles are preferably of a shape having a low uniformity, preferably at least one of a ratio of length, width and thickness dimension being greater than about 1.5, more preferably greater than about 3 The best is above about 5. Preferred shapes for this embodiment are a small piece shape, a rod or needle shape or a small piece shape, a rod or needle shape, and a combination of other shapes.

Another parameter that characterizes the particle size distribution is d ₅₀ . d ₅₀ is the median diameter or median value of the particle size distribution. It is the value of the particle diameter at 50% of the cumulative distribution. The particle size distribution can be measured via laser diffraction, dynamic light scattering, imaging, electrophoretic light scattering, or any other method known in the art. In particular, the particle size of the present invention is determined according to ISO 13317-3:2001. A Microtrac S3500 instrument with the accompanying software (manufactured by Microtrac Corporation of Montgomeryville, Pennsylvania, operating according to X-ray gravity technology) was used for the measurement. Approximately 0.3 grams of the sample was weighed, placed in a 100 ml glass beaker and 2.3 grams of surfactant solution (5% RV-260 in deionized water) was added. The beaker was then filled to 20 ml line with deionized water and stirred by hand. The beaker was placed on a stirrer at 350 rpm for about 1 minute. Using a pipette, the sample is slowly loaded into the circulating fluid in the sample delivery controller and measurement begins. The particle size distribution is determined by software and is given by d ₅₀ .

Preferably, the metal particles have a median particle diameter d ₅₀ of at least about 0.1 μm and preferably at least about 0.5 μm. Meanwhile, d _{50 is} preferably not more than about 5 μm and more preferably not more than about 3.5 μm.

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 can be measured by the BET (Brunauer-Emmett-Teller) method known in the art. In particular, the BET measurement is carried out in accordance with DIN ISO 9277:1995. A Monosorb instrument (manufactured by Quantachrome Instruments), which was operated according to the SMART method with Adaptive dosing Rate, was used for the measurement. According to one embodiment, the metal particles may have a specific surface area of at least about 0.1 m ² /g, preferably at least about 0.2 m ² /g. Meanwhile, the specific surface area is preferably not more than 5 m ² /g and more preferably not more than about 2 m ² /g.

Glass frit

The frit acts as an adhesive medium that facilitates bonding between the electrically conductive particles and the crucible substrate and thereby provides reliable electrical contact. In particular, the frit is etched through a surface layer (eg, an anti-reflective layer) of the tantalum substrate such that there is effective electrical contact between the conductive paste and the tantalum wafer.

According to one embodiment, the electrically conductive paste comprises at least about 0.5 wt% and preferably at least about 1 wt% glass frit, based on 100% total weight of the paste. Also, the paste preferably comprises no more than about 15% by weight, preferably no more than about 10% by weight and most preferably no more than about 6% by weight of the glass frit, based on 100% by weight of the conductive paste.

Preferred glass frits are powders of glass-converted amorphous or partially crystalline solids. The glass transition temperature _Tg is the temperature at which the amorphous material is converted from a rigid solid to a partially moving supercooled melt upon heating. Methods for determining the glass transition temperature are well known in the art. In particular, the glass transition temperature _Tg can be determined using a DSC apparatus SDT Q600 (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 a furnace with a platinum/platinum-rhodium (R-type) thermocouple. The sample holder used was an alumina ceramic crucible having a capacity of about 40-90 μl. For measurement 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 were applied respectively. As the disk for the reference and the sample, an alumina disk having a volume of about 85 μl was used. A sample of about 10-50 mg was weighed into the sample pan with an accuracy of 0.01 mg. Place the blank reference plate and sample plate in the unit, turn off the furnace and start measuring. A heating rate of 10 K/min was used from the initial temperature of 25 ° C to the end temperature of 1000 ° C. The balance in the instrument was always purged with nitrogen (N ₂ 5.0) and the furnace was purged with synthetic air (80% N ₂ and 20% O ₂ from Linde) at a flow rate of 50 ml/min. The first step in the DSC signal was evaluated as glass transition using the above software, and the measured initial value was the temperature of _Tg .

Preferably, the _{Tg is} lower than the desired firing temperature of the conductive paste. Preferably, the glass frit has a _Tg of at least about 200 ° C and preferably at least about 250 ° C in accordance with the present invention. At the same time, the preferred glass frit has a _Tg of no more than about 700 ° C, preferably no more than about 650 ° C and most preferably no more than about 500 ° C.

The glass frit may also include elements, oxides, compounds that form oxides upon heating, and/or mixtures thereof. According to one embodiment, the frit is based on lead and may include lead oxide or other lead-based compounds including, but not limited to, salts of lead halides, lead oxides, lead carbonate, lead sulfate, lead phosphate, nitric acid Lead and organometallic lead compounds or compounds which form lead oxides or salts during thermal decomposition or any combination thereof. In another embodiment, the frit may be free of lead. The term "lead-free" indicates that the glass frit has less than 0.5% by weight lead based on 100% of the total weight of the frit. Lead-free glass frits may include other oxides or compounds known to those skilled in the art including, but not limited to, germanium, boron, Aluminum, bismuth, lithium, sodium, magnesium, zinc, titanium, zirconium oxide or a compound thereof.

In addition to the above components, the glass frit may also contain other compounds for improving the contact properties of the resulting conductive paste. For example, the glass frit may also contain Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, V, Zr, Mo, Mn, Zn, B, P, Sn, Ga, Ge, In, Al. An oxide or other compound of Sb, Bi, Ce, Cu, Ni, Cr, Fe, Co or any combination thereof. Examples of such oxides and compounds include, but are not limited to, cerium oxides, vanadium oxides, molybdenum oxides, cerium oxides, lithium oxides, tin oxides, indium oxides, rare earth oxides (eg, La ₂ O ₃ or cerium oxide), phosphorus oxides, transition metal oxides (such as copper oxides and chromium oxides), metal halides (such as lead fluorides and zinc fluorides), and combinations thereof. The oxides and compounds are preferably present in an amount of at least about 0.1% by weight and not more than about 15% by weight, based on 100% by total weight of the glass frit.

As is well known to those skilled in the art, frit particles can exhibit a variety of shapes, sizes, and surface area to volume ratios. The glass particles can assume the same or similar shape (including length: width: thickness ratio) as can be exhibited by conductive metal particles as discussed herein. Glass frit particles having a combination of shapes or shapes that facilitate improved electrical contact of the electrodes produced are preferred.

Preferably, in a glass frit particle median particle diameter d ₅₀ of at least about 0.1μm. Also, preferably, the glass frit has a d _{50 of} 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 m ² /g, preferably at least about 1 m ² /g and most preferably at least about 2 m ² /g. Also, preferably, the specific surface area does not exceed about 11 m ² /g, preferably does not exceed about 10 m ² /g and most preferably does not exceed about 8 m ² /g.

According to another embodiment, the frit particles may comprise a surface coating. Any such coating known in the art and considered suitable for the context of the present invention can be used on frit particles. Preferred coatings of the present invention include such coatings which promote dispersion of the glass in the organic vehicle and improved contact of the conductive paste. Preferably, if present, the coating corresponds to no more than about 10% by weight, preferably no more than about 8% by weight, most preferably no more than about 5% by weight, in each case based on glass The total weight of the glass particles.

additive

Preferred additives in the context of the present invention are components which are added to the paste in addition to the other components specifically mentioned, which promote the electrical properties of the paste, the electrode it produces or the resulting solar cell. In addition to the additives present in the glass frit and the carrier, the additives may also be present in the conductive paste. Preferred additives include, but are not limited to, thixotropic agents, viscosity modifiers, emulsifiers, stabilizers or pH adjusters, inorganic additives, thickeners and dispersants, or combinations 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), oxidation thereof a compound, a compound capable of forming a metal oxide thereof during firing, a mixture of at least two of the foregoing metals, a mixture of at least two of the foregoing oxides, and at least two of the foregoing compounds of at least two of the metals Mixture or a mixture of two or more of any of the foregoing.

According to one embodiment, the paste may include at least about 0.1 wt% additive. Also, the paste preferably comprises no more than about 10% by weight, preferably no more than about 5% by weight and most preferably no more than about 2% by weight of the additive, based on 100% by weight of the paste.

In one embodiment, the conductive paste is free or substantially free of pigments (eg, black pigments). The term "substantially free" as used herein means that the paste has less than 0.1% by weight pigment based on 100% total weight of the paste.

Forming a conductive paste composition

To form a conductive paste, the frit material and the conductive metal particles and the organic vehicle are combined using any method known in the art for preparing a paste composition. The preparation method is not critical as long as it produces a uniformly dispersed paste. The components can be mixed, for example, with a mixer, and then passed through a three-roll mill to, for example, produce a uniformly dispersed paste. In addition to all The components can also be mixed together with the silver particles in a ball mill for 2 to 24 hours to obtain a homogeneous mixture of glass frit and silver particles, which is then combined with an organic solvent in a mixer.

Solar battery

The invention also relates to a solar cell. In one embodiment, a solar cell comprises a semiconductor substrate (eg, a germanium wafer) and a conductive paste composition of any of the embodiments described herein.

In another aspect, the invention is directed to a solar cell made by a process comprising applying a conductive paste of any of the embodiments described herein to a semiconductor substrate (eg, a germanium wafer) The composition and the fired semiconductor substrate.

Silicon wafer

Preferred wafers of the present invention have regions capable of absorbing light with high efficiency to produce electron-hole pairs and separating holes and electrons across the boundary, preferably across the boundaries of the pn junction, particularly for solar cells. Area. Preferred wafers of the present invention comprise a single body of a front doped layer and a back doped layer.

Preferably, the wafer comprises a suitably doped tetravalent element, a binary compound, a ternary compound or an alloy. Preferred tetravalent elements in this context include, but are not limited to, ruthenium, osmium or tin, preferably ruthenium. Preferred binary compounds include, but are not limited to, a combination of two or more tetravalent elements, a binary compound of a group III element and a group V element, a binary compound of a group II element and a group VI element, or a group IV element. A binary compound with a Group VI element. Preferred combinations of tetravalent elements include, but are not limited to, two or more combinations of elements selected from the group consisting of ruthenium, osmium, tin or carbon, preferably SiC. A preferred binary compound of the group III element and the group V element is GaAs. In accordance with a preferred embodiment of the present invention, the wafer is germanium. The above description, explicitly mentioned, is also applicable to other wafer compositions described herein.

The p-n junction boundary is located where the doped layer and the back doped layer meet before the wafer. In an n-type solar cell, the post-doped layer is doped with an electron-donating n-type dopant and the front doped layer is accepted The electron or power supply hole is doped with a p-type dopant. In a p-type solar cell, the back doped layer is doped with a p-type dopant and the front doped layer is doped with an n-type dopant. In accordance with a preferred embodiment of the present invention, a wafer having a p-n junction boundary is produced by first providing a doped germanium substrate and subsequently applying a doped layer of the opposite type to one face of the substrate.

The doped germanium substrate can be made by any method known in the art and considered suitable for the present invention. Preferred sources of the tantalum substrate of the present invention include, but are not limited to, single crystal germanium, polycrystalline germanium, amorphous germanium, and upgraded metallurgical germanium, most preferably single crystal germanium or polycrystalline germanium. Doping to form a doped germanium substrate can be performed simultaneously by adding a dopant during the preparation of the germanium substrate, or it can be carried out in a subsequent step. The doping after the preparation of the germanium substrate can be carried out by, for example, gas diffusion epitaxy. The doped germanium substrate can also be readily purchased. According to one embodiment, the initial doping of the germanium substrate can be carried out simultaneously with the formation of a dopant by adding a dopant to the germanium mixture. According to another embodiment, the application of the front doped layer and the highly doped back layer, if present, can be performed by vapor phase epitaxy. Preferably, the vapor phase epitaxy is carried out at a temperature of at least about 500 ° C, preferably at least about 600 ° C and optimally at least about 650 ° C. At the same time, the temperature preferably does not exceed about 900 ° C, preferably does not exceed about 800 ° C and most preferably does not exceed about 750 ° C. The vapor 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 time, the pressure preferably does not exceed about 100 kPa, preferably does not exceed about 80 kPa, and most preferably does not exceed about 70 kPa.

It is known in the art that tantalum substrates can take on a variety of shapes, surface textures, and sizes. The shape of the substrate may include a rectangular parallelepiped, a disk, a wafer, and an irregular polyhedron (only a few are listed). In accordance with a preferred embodiment of the present invention, a wafer has two similar, preferably equal, and rectangular parallelepipeds that are significantly smaller than the other two dimensions. The third dimension can be as small as 1/100 for the first two dimensions. Further, a tantalum substrate having a rough surface is preferred. One way to assess the roughness of a substrate is to evaluate the surface roughness parameters of the subsurface of the substrate that is small compared to the total surface area of the substrate, preferably less than about 1/100 of the total surface area and substantially flat. The value of the surface roughness parameter is obtained by projecting the subsurface by the area of the subsurface The ratio of the area of the theoretical surface formed to the flat surface is given, which is best fitted to the subsurface by minimizing the mean square displacement. The higher the value of the surface roughness parameter, the coarser and more irregular the surface is indicated, and the lower the value of the surface roughness parameter, the smoother and flatter the surface is indicated. In accordance with the present invention, the surface roughness of the tantalum substrate is preferably modified to provide an optimum balance between a variety of factors including, but not limited to, light absorption and adhesion to the surface.

The two larger dimensions of the tantalum substrate can be modified to suit the desired application of the resulting solar cell. Preferably, the germanium wafer has a thickness of less than about 0.5 mm, more preferably less than about 0.3 mm, and most preferably less than about 0.2 mm, in accordance with the present invention. Some wafers have a minimum thickness of 0.01 mm or more.

Preferably, the front doped layer is thinner than the back doped layer. Also preferably, the front doped layer has a thickness of at least about 0.1 μm, preferably no more than about 10 μm, preferably no more than about 5 μm, and most preferably no more than about 2 μm.

A highly doped layer can be applied to the back side of the germanium substrate between the back doped layer and any other layers. The highly doped layer and the back doped layer have the same doping type and the layer is typically represented by + (the n+ type layer is applied to the n-type back doped layer and the p+ type layer is applied to the p-type back doped layer). This highly doped backing layer serves to aid in metallization and improve electrical conductivity. Preferably, the highly doped backing layer, if present, has a thickness of at least 1 μm, preferably no more than about 100 μm, preferably no more than about 50 μm and most preferably no more than about 15 μm.

Dopant

Preferred dopants are those that form p-n junction boundaries by introducing electrons or holes into the ribbon structure when added to the germanium wafer. Preferably, the identity and concentration of the dopants are selected to tune the band structure of the p-n junction and to set the desired light absorption and conductivity characteristics. Preferred p-type dopants include, but are not limited to, those that add holes to the germanium wafer ribbon structure. All dopants known in the art and considered suitable for the context of the present invention can be used as p-type dopants. Preferred p-type dopants include, but are not limited to, trivalent elements, specifically those of the 13th group of their periodic tables. Preferred Group 13 elements of the periodic table in this context include, but are not limited to, boron, Aluminum, gallium, indium, antimony or a combination of at least two thereof, of which boron is particularly preferred.

Preferred n-type dopants are those in which electrons are added to the germanium wafer ribbon structure. Preferably, the n-type dopant is a Group 15 element 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 combinations of at least two thereof, with phosphorus being preferred.

As noted above, the various degrees of doping of the p-n junction can be varied to tune the desired properties of the resulting solar cell. The degree of doping was measured using secondary ion mass spectrometry.

According to certain embodiments, a semiconductor substrate (ie, a germanium wafer) exhibits a sheet resistance of greater than about 60 Ω/□ (eg, greater than about 65 Ω/□, 70 Ω/□, 90 Ω/□, or 100 Ω/□). For measuring the sheet resistance of the surface of the doped germanium wafer, a device "GP4-Test Pro" (available from GP Solar GmbH) equipped with a software package "GP-4 Test 1.6.6 Pro" was used. For measurement, the four-point measurement principle is applied. Two external probes apply a constant current and two internal probes measure the voltage. The sheet resistance (Ω/□) was inferred using Ohm's law. To determine the average sheet resistance, measurements were taken at 25 equally distributed points of the wafer. In an air control room at a temperature of 22 ± 1 ° C, all equipment and materials are balanced prior to measurement. To perform the measurement, "GP-Test.Pro" is equipped with a 4-point measuring head (part number 04.01.0018) with a sharp tip to penetrate the anti-reflection and/or passivation layer. Apply 10 mA of current. The probe is brought into contact with the non-metallized wafer material and measurement begins. After measuring 25 equally distributed points on the wafer, the average sheet resistance is calculated in Ω/□.

Solar cell structure

The contribution to achieving at least one of the above objectives is carried out by solar cells obtained from the process of the present invention. The preferred solar cells of the present invention are highly efficient and their light and durable in terms of the ratio of the total energy of the incident light that is converted to electrical energy output. At a minimum, the solar cell includes (i) a front electrode, (ii) a front doped layer, (iii) a p-n junction boundary, (iv) a post doped layer, and (v) a solder pad. Solar cells can also include additional layers for chemical/mechanical protection.

Antireflection layer

According to the present invention, the antireflection layer can be applied as an outer layer before the electrode is applied to the front side of the solar cell. All antireflective layers known in the art and considered suitable for the context of the present invention may be employed. Preferably, the antireflective layer reduces the proportion of incident light reflected from the front and increases the proportion of incident light across the surface to be absorbed by the wafer. The antireflection layer is preferably: produces an advantageous absorption/reflection ratio, is easily etched by the conductive paste, or is resistant to the temperature required for the conductive paste to be fired, and does not promote the recombination of electrons and holes near the electrode interface. . Preferred antireflective layers include (but are not limited _{to) SiN x, SiO 2, Al} 2 O 3, TiO 2 or a mixture of at least two of, and / or a combination of at least two layers. According to a preferred embodiment, the anti-reflection layer system SiN _x, where employed specifically silicon wafers.

The thickness of the antireflective layer is adapted to the wavelength of the appropriate light. According to a preferred embodiment of the invention, the antireflection layer has 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 layer

One or more passivation layers may be applied to the front side and/or the back side of the germanium wafer as an outer layer. The passivation layer can be applied prior to forming the front electrode or before applying the anti-reflective layer, if any. Preferred passivation layers are those which reduce the ratio of electron/hole recombination near the electrode interface. Any passivation layer known in the art and considered suitable for the context of the present invention can be employed. Preferred passivation layers of the present invention include, but are not limited to, tantalum nitride, hafnium dioxide, and titanium dioxide. According to a more preferred embodiment, tantalum nitride is used. The passivation layer preferably has a thickness of at least 0.1 nm, preferably at least 10 nm and most preferably at least 30 nm. At 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.

Extra layer

In addition to the above layers, other layers may be added for mechanical and chemical protection. The battery can be encapsulated to provide chemical protection. According to a preferred embodiment, a transparent polymer (commonly referred to as a transparent thermoplastic resin) is used as the encapsulating material if the encapsulation is present. Better transparency in this context The polymer is ruthenium rubber and polyvinyl acetate (PVA). A transparent glass sheet can also be added to the front of the solar cell to provide mechanical protection to the front side of the battery. A back protection material can be added to the back of the solar cell to provide mechanical protection. Preferred back protection materials are those which have good mechanical properties and weather resistance. A preferred back protective material of the present invention is polyethylene terephthalate of a layer of polyvinyl fluoride. The back protection material is preferably present under the encapsulation layer (provided both the back protection layer and the encapsulation are present).

A frame material can be added to the exterior of the solar cell to create a mechanical support. Frame materials are well known in the art and any frame material deemed suitable for the context of the present invention can be employed. A preferred frame material for the present invention is aluminum.

Method of preparing solar cell

The solar cell can be produced by applying the conductive paste of the present invention (for example, tantalum nitride, hafnium oxide, titanium oxide or aluminum oxide) to the antireflection coating on the front side of a semiconductor substrate such as a tantalum wafer. A backside conductive paste is then applied to the back side of the solar cell to form a solder pad. The aluminum paste is then applied to the back side of the substrate such that the edges of the solder pads formed from the backside conductive paste overlap to form the BSF.

The electrically conductive paste can be applied in any manner known in the art and deemed suitable for the context of the present invention. Examples include, but are not limited to, dipping, immersing, pouring, immersing thereon, injecting, 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, pad printing, lithographic printing, letterpress printing or stencil printing, or a combination of at least two thereof. According to the invention, the conductive paste is preferably applied by printing, preferably by screen printing. In particular, the screen preferably has a mesh opening having a diameter of about 40 [mu]m or less (e.g., about 35 [mu]m or less, about 30 [mu]m or less). At the same time, the screen preferably has a mesh opening having a diameter of at least 10 μm.

The substrate is then fired according to the appropriate curve. A firing is required to sinter the printed conductive paste to form a solid electrode. Firing is well known in the art and can be considered in any manner suitable for the context of the present invention. Preferably, the firing may be performed at higher T _g the glass frit materials.

In accordance with the present invention, the maximum temperature setting for firing is less than about 900 ° C, preferably less than about 860 ° C. A firing temperature as low as about 800 ° C has been used to obtain a solar cell. The firing temperature should allow for effective sintering of the metal particles. The firing temperature profile is typically set to burn off the organic material of the conductive paste composition. The firing step is typically carried out in a belt furnace in air or in an oxygen-containing atmosphere. The firing is preferably carried out in a rapid 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 preferably in the range of about 3 to 7 seconds. The substrate can achieve a peak temperature in the range of about 700 ° C to 900 ° C for a period of about 1 to 5 seconds. Firing can also be performed at a high delivery rate (e.g., about 100-700 cm/min) with a resulting hold time of about 0.5 to 3 minutes. Multiple temperature zones (eg, 3 to 12 zones) can be used to control the desired thermal profile.

The firing of the conductive paste on the front and back sides can be carried out simultaneously or sequentially. If the conductive paste applied to both faces has similar (preferably the same) optimum firing conditions, the firing is appropriate at the same time. If appropriate, it is preferred to carry out the firing simultaneously. When the firing is performed sequentially, it is preferred to first apply and fire the back conductive paste, and then apply a conductive paste to the front surface of the substrate and fire it.

Measuring properties of conductive paste

The sample solar cell was characterized using an IV-tester "cetis PV-CTL1" available from Halm Elektronik GmbH. During the power measurement, all parts of the measuring equipment and solar cells to be tested were maintained at 25 °C. This temperature should be measured simultaneously by the temperature probe on the surface of the battery during actual measurement. The Xe arc lamp stimulates daylight on the surface of the battery with a known AM 1.5 intensity of 1000 W/m ² . In order for the stimulator to reach this intensity, the lamp is flashed several times in a short period of time until a stable value is reached by the "PVCTControl 4.313.0" software monitoring of the IV-tester. The Halm IV tester uses a multi-point contact method to measure current (I) and voltage (V) to determine the IV-curve of the solar cell. To this end, the solar cells are placed between the multi-point contact probes in such a way that the probe fingers contact the busbars (ie, printed lines) of the solar cells. The number of contact probe wires is adjusted to the number of bus bars on the surface of the battery. All electrical values are automatically determined from this curve directly by the executed software package. As a reference standard, calibrated solar cells from ISE Freiburg and consisting of the same area dimension, the same wafer material and treated with the same front side outer layer were tested and the data were compared to nominal values. The at least 5 wafers processed in the same manner are measured and interpreted by calculating the average of each value. The soft PVCTControl 4.313.0 provides values for efficiency, fill factor, short circuit current, series resistance and open circuit voltage.

To measure the viscosity of the conductive paste, a Brookfield HBDV-III digital rheometer equipped with a CP-44Y sample cup and a No. 51 cone was used. The temperature of the sample was maintained at 25 ° C using a TC-502 circulating warm bath. The measurement gap was set to 0.026 mm with a sample volume of about 0.5 ml. The sample was equilibrated for 2 minutes and then a constant rotational speed of 1.0 rpm was applied for 1 minute. The viscosity of the sample after this interval is reported in units of kcp.

Solar battery module

The plurality of solar cells of the present invention may be spatially arranged and electrically connected to form a collective arrangement called a module. The preferred module of the present invention can have a variety of arrangements, preferably referred to as a rectangular arrangement of solar panels. A variety of ways of electrically connecting solar cells and mechanically arranging and securing the cells to form a collective arrangement are well known in the art. Preferred methods of the present invention are those that produce low mass to power output ratios, low volume to power output ratios, and high durability. Aluminum is a mechanically preferred material for the solar cell of the present invention.

In one embodiment, a plurality of 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 the output line. Solar cells are typically encapsulated in a clear thermoplastic resin such as silicone rubber or ethylene vinyl acetate. A clear glass sheet is placed on the surface of the front surface of the encapsulated transparent thermoplastic resin. A back protective material (for example, a polyethylene terephthalate coated polyethylene terephthalate sheet) is placed under the encapsulated thermoplastic resin. The stratified materials can be heated in a suitable vacuum oven to remove air and then integrated into one by heating and pressing. In addition, because solar cells are usually in open air After being left for a long time, it is desirable to cover the periphery of the solar cell with a frame material composed of aluminum or the like.

The invention will now be illustrated in conjunction with the following non-limiting examples.

Instance

By combining about 33 wt% technosol, about 33 wt% diethylene glycol monobutyl ether solvent, about 16.5 wt% diethylene glycol monobutyl ether acetate solvent, about 0.5 wt% surfactant, about 12 wt% An exemplary organic vehicle was prepared with a thixotropic agent and about 5 wt% acrylic resin (Elvacite ^® 2045 manufactured by Lucite International). All weight percentages are based on 100% total weight of the organic vehicle. The mixture was heated to a temperature of 65 ° C while stirring and then maintained at this temperature for a total of 30 minutes. The carrier was then cooled and ground using a three roll mill until it reached uniformity.

An exemplary conductive paste is then prepared by mixing about 9 wt% of the above organic vehicle, 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 homogeneous paste.

The viscosity and solid loading of the exemplary paste and the control paste (no acrylic resin) were then measured according to the parameters described herein. The solids loading is based on the amount of solids (i.e., glass and conductive particles) in the paste composition based on 100% by weight of the paste. The measurements are set forth in Table 1 below.

Subsequently, an exemplary paste and a control were applied at a speed of 150 mm/s using a screen 325 (mesh) * 0.9 (mil, wire diameter) * 0.6 (mil, emulsion thickness) * 40 μm (line opening) (Calendar wire mesh) at a speed of 150 mm/s. The paste is screen printed onto the wafer. The amount of paste deposited on the tantalum wafer (before drying and firing) was also measured and is set forth in Table 1. The printed wafer was then dried at 150 ° C and fired in a linear multi-zone infrared oven for a few seconds at a peak temperature of about 800 ° C.

The printed finger is then photographed and measured for analysis. The height and width are measured along the length of each finger line as described in Table 2 below, and the average, median, minimum, and maximum values for each measurement are calculated. The lines were measured using a Zeta-200 optical profilometer manufactured by Zeta Instruments of San Jose, California. The aspect ratio is also calculated by dividing the average height by the average width.

Exemplary pastes exhibit a higher aspect ratio, as evidenced by higher average heights and lower average widths.

The resulting solar cells were then tested using the I-V tester according to the parameters described herein. Both the control paste and the exemplary paste include Eta (efficiency), short circuit current density (Isc), open circuit voltage (Voc), fill factor (FF), series resistance (Rs3) at three standard illumination intensities, and shunt The electrical performance characteristics of the resistor (Rsh) are provided in Table 3 below. All data for the control paste and the exemplary paste were normalized to 1. The normalized data of the experimental paste was calculated by dividing the normalized control battery by the relevant exemplary measurement.

Finger lines printed with exemplary pastes demonstrate improved electrical performance compared to lines printed with control paste. The exemplary paste exhibited increased efficiency, short circuit current density, and open circuit voltage compared to the control paste. It is believed that the larger average height of the exemplary lines and the reduced average width (higher aspect ratio) contribute to improved electrical performance.

These and other advantages of the present invention will become apparent to those skilled in the art from this description. Therefore, those skilled in the art will recognize that the above-described embodiments may be modified or modified without departing from the broader inventive concept of the invention. The specific dimensions of any particular embodiment are set forth for illustrative purposes only. Therefore, it is understood that the invention is not limited to the specific embodiments described herein, but is intended to include all modifications and changes in the scope and spirit of the invention.

Claims (20)

A conductive paste composition for producing a solar cell, comprising: (a) conductive metal particles; (b) a glass frit; and (c) an organic vehicle comprising (i) an organic solvent, and Ii) an acrylic resin having no active hydrogen functional group. The conductive paste composition of claim 1, wherein the conductive metal particles are at least 35 wt%, preferably at least 50 wt%, more preferably at least 70 wt%, and most preferably at least 80 wt% based on 100% by total weight of the paste. % and no more than about 99% by weight, preferably no more than about 95% by weight. The conductive paste composition of claim 1 or 2, wherein the conductive metal particles are at least one of silver, gold, copper, aluminum, nickel, and mixtures or alloys thereof, preferably silver. The conductive paste composition according to any one of the preceding claims, wherein the glass frit is at least about 0.5% by weight, preferably at least about 1% by weight and not more than about 15% by weight, based on 100% by total weight of the paste. Preferably no more than about 10% by weight and most preferably no more than about 6% by weight. The conductive paste composition according to any one of the preceding claims, wherein the organic vehicle is at least about 1% by weight, more preferably at least about 5% by weight and not more than about 20% by weight, based on 100% by total weight of the paste. Preferably no more than about 15% by weight. The conductive paste composition according to any one of the preceding claims, wherein the organic vehicle comprises at least about 1% by weight, preferably at least about 2% by weight and most preferably at least about 4% by weight, based on 100% by total weight of the organic vehicle. Resin and no more than about 15% by weight, preferably no more than about 12% by weight and most preferably no more than about 10% by weight of acrylic resin. The conductive paste composition according to any one of the preceding claims, wherein the acrylic resin is an isobutyl methacrylate resin. The conductive paste composition of any of the preceding claims, wherein the acrylic resin has a minimum of 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 optimally no more than about 250 KD. The molecular weight. The conductive paste composition according to any of the preceding claims, wherein the organic vehicle further comprises a thixotropic agent. The conductive paste composition of claim 9, wherein the organic vehicle comprises at least about 5 wt% and preferably at least about 8 wt% and not more than about 15 wt% and preferably no more than about 100% by weight based on the total weight of the organic vehicle. 13wt% thixotropic agent. The conductive paste composition according to any one of the preceding claims, wherein the organic vehicle further comprises at least about 0.01% by weight of surfactant and no more than about 10% by weight, preferably no, based on 100% by total weight of the organic vehicle. More than about 5% by weight and optimally no more than about 3% by weight of surfactant. The conductive paste composition according to any one of the preceding claims, wherein the organic vehicle comprises at least about 60% by weight, preferably at least about 70% by weight and most preferably at least about 80% by weight, based on 100% by total weight of the paste. The solvent does not exceed about 90% by weight of the solvent. The conductive paste composition according to any one of the preceding claims, wherein the organic solvent comprises texanol, diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate or any combination. The conductive paste composition according to any of the preceding claims, wherein the organic vehicle does not comprise a urethane acrylate resin. A conductive paste composition according to any of the preceding claims, wherein the paste composition has a viscosity of at least about 50 Kcp and at most about 300 Kcp. A conductive paste composition according to any of the preceding claims, wherein the paste composition is free or substantially free of pigment. A conductive paste composition according to any one of the preceding claims, wherein the paste composition is capable of screen printing via a screen opening having a diameter of about 40 μm or less. A solar cell produced by applying a conductive paste as claimed in claims 1 to 17 to a germanium wafer and firing the germanium wafer. A method of producing a solar cell, comprising the steps of: providing a tantalum wafer having a front side and a back side; applying a conductive paste according to any one of claims 1 to 17 to the tantalum wafer; and firing the crucible Wafer. A method of producing a solar cell according to claim 19, wherein the conductive paste is applied to the front side of the germanium wafer.