Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
  • H01B1/14—Conductive material dispersed in non-conductive inorganic material
  • H01B1/16—Conductive material dispersed in non-conductive inorganic material the conductive material comprising metals or alloys
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F10/00—Individual photovoltaic cells, e.g. solar cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy

Definitions

• the present invention relates to a thick film conductive composition used for the formation of electrodes in the production of solar cells.
• a component of conductive compositions used in such applications is a glass powder added to achieve sufficient adhesive strength to the substrate during a short firing period.
• Typical conductive metal powders such as silver powder, are used.
• the conductive metal is mixed with a glass powder (glass frit), various additives and organic medium.
• Typical examples for promoting the sintering effect and lowering the firing temperature in thick-film composition electrodes include electrically conductive solar cell compositions composed of a silver powder and at least one type of metal selected from vanadium, molybdenum and tungsten, or a compound thereof as described in JP-A 10-326522.
• a conventional conductive composition for forming the thick-film electrodes on the n-type semiconductor substrate having a thin film for antireflection and passivation requires a glass powder as one component of the formulation, which is dissolved and breaks through the thin-film to form an electrical connection to the semiconductor upon firing.
• the conductive paste composition comprises a glass composition that includes cadmium or an oxide thereof, a relatively good adhesive strength between the electrodes and substrate is achieved.
• a conductive paste composition comprising glass compositions without such cadmium or an oxide thereof are considered as being environmentally desirable but they cannot provide a sufficient adhesive strength between the electrodes formed therefrom.
• Such glass powders soften and flow during firing, and tend to segregate at the electrode/substrate boundary and on the electrode surface where they form an insulating layer that may increase the contact resistance of the electrode. Reducing the amount of glass powder or adding or lowering the firing temperature in order to prevent segregation has the undesirable effect by markedly reducing the adhesive strength of the electrode. In cases where the aforementioned antireflection thin film is formed on the substrate surface, reducing the amount of the glass powder addition or lowering the firing temperature results in insufficient removal of such thin film causes an increase in contact resistance.
• conductive compositions comprising at least one metal powder selected from silver, copper and nickel, and a crystalline complex oxide comprising at least one element selected from bismuth, iron and silver and at least one Group N or Group VI element from the periodic table, which enables the formation of electrodes having a low contact resistance and a high adhesive strength are disclosed in JP-A 11-329070. Given the existence of such a trade-off between lowering the contact resistance and increasing the adhesive strength, it has not been possible to simultaneously achieve both a low contact resistance and a high adhesive strength using conductive compositions that are presently available. It is the object of the invention to provide a thick film conductive composition for forming electrodes for use in solar cells that have a low contact resistance and also have a high adhesive strength even though the glass powders contained in the conductive composition contains no cadmium oxide.
• the invention is directed to a silver conductor composition comprising silver powder, Cd-free glass powder, and 0.1 to 10.0 wt %, based on total composition, particles of at least one metal oxide powder being selected from the group Fe 2 O 3 , FeO, MnO, and CU 2 O wherein all powders are dispersed in an organic medium.
• the invention is also directed to an electrode comprising the above composition disposed on a silicon substrate wherein the composition has been fired and wherein the silver powder, glass powder and metal oxides are sintered and organic medium is removed.
• the invention includes a conductive composition for forming electrodes on a substrate for use in solar cells.
• the composition comprises silver powder, a Cd-free glass powder, and 0.1 to 10.0 wt%, based on a total weight of the composition, of metal oxide powder selected from the group Fe 2 O 3 , FeO, MnO, and Q12O in the form of finely divided solids dispersed in an organic medium.
• metal oxide powder selected from the group Fe 2 O 3 , FeO, MnO, and Q12O in the form of finely divided solids dispersed in an organic medium.
• it is the conductive composition which is characterized by using Fe oxide as the metal oxide.
• the electrodes formed of the conductive paste composition of this invention has excellent adhesive strength for the electrode made therefrom and the substrate and avoids increased contact resistance of the electrode with the substrate.
• the composition of the invention When the conductive composition of the invention is used to form an ohmic electrode on an n-type semiconductor, the composition is generally prepared so that it can be formed into a thick film composition electrode by coating onto an n-type semiconductor substrate such as a silicon wafer using screen printing or other suitable processes, then firing, for example, in a near- infrared oven wherein electromagnetic radiation has relatively shorter wavelengths, for example, in the range from 0.75 to 2.5 micron.
• the conductive composition according to the invention can be used to form solar cell electrodes having low contact resistance and high bond strength. This makes it possible to provide reliable and highly durable solar cells.
• Silver in the form of either flakes or a powder can be used to practice the invention.
• the particle size is not critical for the present invention from the standpoint of technical effects. However, the particle size does have an influence on the sintering properties of the silver powders, with larger particles resulting in a slower sintering speed than small particles.
• the silver particles must be of a size suitable for the method of application to the substrate (generally screen-printing). Accordingly, the size of the silver particles should fit through the screen openings, typically not exceeding 10 microns, and preferably not exceeding 5 microns.
• the silver is generally of high purity (above 99%). However, silver of a lower purity may be used to satisfy the electrical requirements of the electronic conductive pattern of electrodes.
• the amount of silver powders in the composition is generally from 60 to 99 wt %, based on the solids (that is, excluding the liquid organic medium), and 50 to 90 wt %, based on the total weight of the composition.
• the conductive metal particles are substantially to totally sintered depending on the firing profile.
• the inorganic binder In order for the composition to be fired at 580 to 800°C, and for suitable sintering, wetting and bonding to the glass substrate to take place, the inorganic binder is a cadmium-free glass frit having a softening point of 350 to 700°C.
• the glass frit used in the present invention aids in sintering the silver particles to promote adhesion of the metal to the substrate upon firing and may be any well known composition, and the binder is selected according to principles well know in the art, and the chemical nature of the glass frit binder is not critical to the function of the invention.
• Lead silicates are widely used in the art, and may be used to work the present invention.
• Lead silicates and modified lead silicates such as lead borosilicate, lead aluminosilicates glasses are highly suitable from the standpoint of both the range of the softening points and the adhesion properties of the glass.
• Minor components of the frit other than PbO and SiO 2 may include TiO 2 , B 2 O 3 , Bi 2 O 3 and Al 2 O 3 .
• the total amount of inorganic binder in the composition is generally 1 to
• softening point refers to the softening point obtained by the fiber elongation method described in ASTM C338-57.
• the inorganic binder particles are substantially to totally sintered depending on the firing profile.
• the metal oxide used in the invention is at least one selected from the group of Fe 2 O 3 , FeO, MnO, and Cu 2 O. Iron oxide is preferred. To obtain a sufficient adhesive strength, the content of metal oxide is from 0.1 to 10 wt %, preferably 0.1 to 5.0%, and more preferably from 0.3 to 1.5%, based on total composition. Similarly, mixture of metal oxides may be used.
• the particle size should be of a size appropriate to the manner in which the composition of the invention is applied, which is usually by screen printing. Thus the particle size should be in about the same range as the conductive powder.
• metal oxide may not give optimum sintering. Therefore, preferably not more than 5 wt %, is preferred. These oxides are important to give the composition low contact resistance and a high adhesive strength.
• the metal oxide particles are substantially to totally sintered depending on the firing profile.
• a suitable inert liquid can be used as the organic medium (vehicle).
• the use of an inert, non-aqueous liquid is preferred.
• Use may be made of any of various organic liquids that may or may not contain a thickener, stabilizer and/or other common additives.
• Illustrative examples of organic liquids that may be used for this purpose include alcohols or alcohol esters (e.g., acetic acid and propionate), terpenes (e.g., pine oil, terpineol), solutions of resins (e.g., polymethacrylate), solutions of ethyl cellulose in a solvent such as pine oil, and the monobutyl ether of ethylene glycol monoacetate.
• the medium may contain volatile liquids to promote high-speed setting after application to the substrate.
• a preferred organic medium is ethyl cellulose in terpineol (1 :9), in combination with a thickener blended with butyl carbitol acetate.
• a composition (paste) can be effectively produced on a three-roll mill.
• the preferred viscosity for these compositions is about 50 to 300 Pas, as measured at 10 rpm and 25°C with a Brookfield HBT viscometer using a #14 spindle and a utility cup.
• the amount of thickener used depends on the viscosity of the final composition; that is, on the conditions required for deposition on a substrate; for example, printing.
• Preferred compositions will contain, complementally, 5 to 50 wt % organic medium and 50 to 95 wt% solids.
• the organic medium is substantially to totally removed upon firing of the composition. Test Method:
• a conductive composition was printed and fired on the light-receiving side of a solar cell substrate so as to form a silver electrode having a film thickness of 10 ⁇ m and a size of 10 x 7 mm.
• copper wires width, 1 mm; thickness, 100 ⁇ m; solder-plated
• the adhesive strength was measured using a tensile testing machine manufactured by Instron Corp. (model 1000).
• Preferred glasses are lead silicates, especially lead calcium aluminoborosilicates; zinc borosilicates; and lead zinc borosilicates; etc.
• Experimental glasses used in Comparative Examples 1 to 6 and Example 7 of Table 1 are as follows:
• Cd glass (experimental product No. F2909 made by E.I. du Pont de Nemours and Company): primary glass-forming oxides include PbO, SiO 2 , B O 3 , TiO 2 and CdO and a softening point is 550°C.
• Experimental Glass A (experimental product No. F2967 made by E.I. du Pont de Nemours and Company): primary glass-forming oxides include PbO, SiO 2 , B O 3 , TiO 2 and CdO and a softening point is 550°C.
• Experimental Glass A (experimental product No. F2967 made by E.I. du
• primary glass-forming oxides include 40-70 wt% and preferably 45-60 wt% PbO, 5-25 wt% SiO 2 , and 0-5 wt% Al 2 O 3 and a softening point is 551°C.
• Experimental Glass B (experimental product No. F2870 made by E.I. du Pont de Nemours and Company): primary glass-forming oxides include 10-20 wt% PbO, 1-10 wt% SiO 2 , 1-10 wt% B 2 O 3 and 65-85wt% Bi 2 O 3 and a softening point is 350°C.
• Example 7 Example 7:
• An organic medium (terpineol containing 20 parts by weight of ethylene cellulose) was added to a mixture of 2.5 parts of Glass A, 0.5 parts of Fe 2 O 3 and 75 parts of silver powder, following which the components were pre-mixed in a general-purpose mixer. The mixture was then blended on a three-roll mill, giving

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• Chemical & Material Sciences (AREA)
• Dispersion Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Conductive Materials (AREA)
• Photovoltaic Devices (AREA)

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Citations (4)

• 2002
  • 2002-09-18 KR KR1020047004021A patent/KR100584073B1/ko not_active Expired - Fee Related
  • 2002-09-18 WO PCT/US2002/029574 patent/WO2003025954A1/en not_active Ceased
  • 2002-09-18 JP JP2003529485A patent/JP4291146B2/ja not_active Expired - Fee Related
  • 2002-09-18 DE DE60212439T patent/DE60212439T2/de not_active Expired - Lifetime
  • 2002-09-18 EP EP02798996A patent/EP1451830B1/en not_active Expired - Lifetime

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