Conductive paste, method of preparation, and solar cell electrode using the same

A method for preparing a conductive paste, the method being characterized by producing hydrophobic spherical silica fine particles that are at least partially surface-coated with an organophosphorus compound and have an average particle size for primary particles thereof, expressed as the volume base median diameter, of from 0.01 to 5 μm, and mixing the organophosphorus compound-coated hydrophobic spherical silica fine particles, in an amount of from 0.1 to 7.0 wt % based on the total weight of the conductive paste, together with conductive particles, a binder, an organic solvent and a glass powder, wherein the organophosphorus compound-coated hydrophobic spherical silica fine particles are produced by a method comprising the steps of:

• (A1) obtaining a mixed-solvent dispersion of SiO2 unit-containing hydrophilic spherical silica fine particles by hydrolyzing and condensing, within a mixture of a hydrophilic organic solvent and water and in the presence of a basic substance, a tetrafunctional silane compound of general formula (I) below
• Si(OR3)4 (I),
• wherein each R3 is independently a monovalent hydrocarbon croup of 1 to 6 carbon atoms, a partial hydrolyzate of the tetrafunctional silane compound, or a mixture thereof;
• (A2) obtaining a mixed-solvent dispersion of hydrophobic spherical silica fine particles of a first type, referred to herein as “surface -hydrophobized spherical silica fine particles,” by adding to the mixed-solvent dispersion of hydrophilic spherical silica fine particles a trifunctional silane compound of general formula (II) below
• R1Si(OR4)3 (II),
• wherein R1 is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 20 carbon atoms and each R2 is independently a monovalent hydrocarbon group of 1 to 6 carbon atoms, a partial hydrolyzate of the trifunctional silane compound, or a mixture thereof and surface-treating the hydrophilic spherical silica fine particles so as to introduce R1SiO3/2 units, wherein R1 is as defined above, onto surfaces of the hydrophilic spherical silica fine particles;
• (A3) obtaining a concentrated mixed-solvent dispersion of the surf ace-hydrophobized spherical silica fine particles by removing part of the hydrophilic organic solvent and the water from, and thus concentrating, the mixed-solvent dispersion of surface -hydrophobized spherical silica fine particles;
• (A4) obtaining hydrophobic spherical silica fine particles of a second type, referred to herein as “hydrophobic spherical silica fine particles, ” by adding a silazane compound of general formula (III) below
• R23SiNHSiR23 (III),
• wherein each R2 is independently a substituted or unsubstituted monovalent hydrocarbon group of 1 to 6 carbon atoms, a monofunctional silane compound of general formula (IV) below
• R23SiX (IV),
• wherein R2 is as defined above and X is a hydroxyl group or a hydrolyzable group, or a mixture thereof to the concentrated mixed-solvent dispersion of surface-hydrophobized spherical silica fine particles and treating the surface-hydrophobized spherical silica fine, particles so as to introduce R23SiO1/2 units, wherein R3 is as defined above, onto the surfaces of the surf ace-hydrophobized spherical silica fine, particles.;
• and
• (A5) obtaining hydrophobic spherical silica fine particles of a third type, referred to herein as “organophosphorus compound-coated hydrophobic spherical silica fine particles,” by dissolving an organophosphorus compound in a dispersion of the hydrophobic spherical silica fine particles so as to at least partially surface-coat the hydrophobic spherical silica fine particles with the organophosphorus compound.