CONDUCTIVE THICK FILM PASTE FOR SOLAR CELL CONTACTS

Abstract

The present invention relates to an inorganic reaction system used in the manufacture of electroconductive pastes. The inorganic reaction system comprises a lead containing matrix forming composition and a tellurium oxide additive. Preferably the lead containing matrix forming composition is between 5-95 wt. % of the inorganic reaction system, and the tellurium oxide additive is between 5-95 wt. % of the inorganic reaction system. The lead containing matrix forming composition may be a glass frit, and may comprise lead oxide. Another aspect of the present invention relates to an electroconductive paste composition that comprises metallic particles, an inorganic reaction system as previously disclosed, and an organic vehicle. Another aspect of the present invention relates to an organic vehicle that comprises one or more of a binder, a surfactant, a solvent, and a thixatropic agent. Another aspect of the present invention relates to a solar cell printed with an electroconductive paste composition as disclosed, as well as an assembled solar cell module. Another aspect of the present invention relates to a method of producing a solar cell.

Claims

1-10. (canceled)

11. An electroconductive paste composition comprising: metallic particles; an inorganic reaction system comprising a lead containing matrix forming composition and a tellurium oxide additive, wherein the lead containing composition is between 5-95 wt % of the inorganic reaction system, and the tellurium oxide additive is between 5-95 wt % of the inorganic reaction system; and an organic vehicle.

12. The electroconductive paste as in claim 11, wherein the metallic particles are at least one of silver, gold, copper, and nickel.

13. The electroconductive paste as in claim 11, wherein the metallic particles are silver.

14. The electroconductive paste as in claim 11, wherein the metallic particles are about 50-95 wt. % of solid content of the paste.

15. The electroconductive paste as in claim 11, wherein the organic vehicle comprises a binder, a surfactant, an organic solvent, and a thixatropic agent.

16-22. (canceled)

23. A solar cell produced by applying an electroconductive paste according to claim 11 to a silicon wafer and firing the silicon wafer according to an appropriate profile.

24. The solar cell of claim 23, wherein the silicon wafer is of sheet resistance above 60 Ω/□.

25-28. (canceled)

29. A solar cell module comprising electrically interconnected solar cells as in claim 23.

30. A method of producing a solar cell, comprising the steps of: a. providing a silicon wafer; b. applying an electroconductive paste according to claim 11 to the silicon wafer; c. firing the silicon wafer according to an appropriate profile.

31. The method of producing a solar cell according claim 30, wherein the silicon water comprising an antireflective coating.

32. The method of producing a solar cell according claim 30, wherein the electroconductive paste according to claim 11 is applied to the light receiving surface of the silicon wafer.

33. The electroconductive paste composition of claim 11, wherein the lead containing matrix forming composition is between 10-90 wt. % of the inorganic reaction system, and the tellurium oxide additive is between 10-60 wt. % of the inorganic reaction system.

34. The electroconductive paste composition of claim 1 1, wherein the tellurium oxide additive comprises one or more of tellurium dioxide, tellurium trioxide, and any tellurium compound that would convert to tellurium oxide at temperature 200-1000′ C.

35. The electroconductive paste composition of claim 11 wherein the tellurium oxide additive is of an average particle size of less than 10 μM.

36. The electroconductive paste composition of claim 11 wherein the tellurium oxide additive is of an average particle size of less than 1 μM.

7. The electroconductive paste composition of claim 11, wherein the lead containing matrix forming composition is a glass frit, preferably with an amorphous structure, and may also incorporate crystalline phases or compounds.

38. The electroconductive paste composition of claim 11, wherein the lead containing matrix forming composition comprises lead oxide.

39. The electroconductive paste composition of claim 11, wherein the lead containing matrix forming composition comprises between about 10-90 wt. %, preferably about 25.85 wt. % lead oxide.

40. The electroconductive paste composition of claim 1, wherein the lead containing matrix forming composition comprises between about 5-45 wt. %, preferably about 10-15 wt. % lead oxide.

41. The electroconductive paste composition of claim 11, wherein the inorganic reaction system has a PbO:Tellurium oxide additive weight percentage ratio of 95:5 to 5:95, preferably the inorganic reaction system has a PbO:Tellurium oxide additive weight percentage ratio of 10:1 to 1:10, and more preferably, the PbO:Tellurium oxide additive weight percentage ratio is 5:1 to 1:5.

Description

DETAILED DESCRIPTION

[0025] The present invention relates to electroconductive paste compositions as used in the manufacturing of solar cells. Electroconductive pastes typically comprise metallic particles, glass frit (an amorphous or partially crystalline material), and an organic vehicle. While not limited to such an application, such pastes may be used to form an electrical contact layer or electrode on a solar cell. Specifically, the pastes may be applied to the front side of a solar cell or to the back side of a solar cell and provide the path by which conductivity occurs between cells.

[0026] One aspect of the present invention provides an inorganic reaction system (IRS). The IRS of the present invention provides a delivery media for the metallic particles, allowing them to migrate from the paste to the interface of the metal conductor and the semiconductor substrate. The IRS of the present invention also provides a reaction media for the paste components to undergo physical and chemical reactions at the interface. Physical reactions include, but are not limited to, melting, dissolving, diffusing, sintering, precipitating, and crystallizing. Chemical reactions include, but are not limited to, synthesis (forming new chemical bonds) and decomposition, reduction and oxidation, and phase transitioning. Lastly, the IRS of the present invention acts as an adhesion media that provides the bonding between the metal conductor and the semiconductor substrate, thereby securing reliable electrical contact performance during the lifetime of the solar device. Although intended to achieve the same effects, existing glass frit compositions can result in high contact resistance due to the insulating effect of the glass in the interface of the metallic layer and silicon wafer. The IRS of the present invention acts as a delivery, reaction, and adhesion media, but provides much lower contact resistance and higher overall cell efficiency.

[0027] More specifically, the IRS of the present invention provides improved Ohmic and Schottky contact between the metal conductor (e.g., silver) and the semiconductor emitter (e.g., silicon substrate) in the solar cell. The IRS of the present invention is a reactive media with respect to the silicon and creates active areas on the silicon emitter that improve overall contact mechanisms, such as through direct contact, or tunneling. The improved contact properties provide better Ohmic contact and Schottky contact, and therefore better overall solar cell performance.

[0028] The IRS of the present invention may comprise crystalline or partially crystalline materials. It may comprise various compounds including, but not limited to, oxides, salts, fluorides, and sulfides, as well as alloys, and elemental materials.

[0029] The preferred embodiment of the present invention relates to an IRS as used in an electroconductive paste that comprises a lead containing matrix forming composition and a tellurium oxide additive. The matrix forming composition fuses or sinters at the firing temperature of the present invention IRS and/or the electroconductive paste comprising an IRS according to the present invention. The matrix forming composition may be a glass, ceramic, or any compounds known to one skilled in the art that can form a matrix at elevated temperature. A preferred embodiment of the lead containing matrix forming composition is a lead containing glass frit. More preferably, a glass frit comprises lead oxide as a starting material. The lead containing matrix forming composition is between 5-95 wt. % of the IRS, more preferably between 25-60 wt. % of the IRS. Further, the lead containing matrix forming composition comprises about 5-95 wt. %, preferably about 10-90 wt. %, more preferably 25-85 wt. %, and even more preferably about 45-75 wt. %, lead oxide. In another embodiment, the lead matrix forming composition may contain a relatively low lead content, e.g., between about 5-45 wt. %, preferably about 10-40 wt. %, and more preferably about 10-15 wt. %, lead oxide.

[0030] Used in the context of the present invention, the term additive refers to a component of the IRS that is discrete, particularly not part of a matrix forming composition. An additive is provided directly to the IRS. In the preferred embodiment, where the lead containing matrix forming composition is a lead containing glass frit, the tellurium oxide additive is not a part of the lead containing glass frit.

[0031] The inclusion of the tellurium oxide additive greatly improves contact with the semiconductor emitter, and reduces the serial resistance. Tellurium oxide as an additive performs a totally different thermal dynamic reaction on a silicon wafer comparing to lead containing tellurite glass, compounds or composition. Tellurium oxide has high reactivity with silicon. The reaction between TeO.sub.2 and Si has a Gibbs free energy change at 1000 K of ΔG=−140.949 Kcal/mol. The reaction between PbO and Si has a AG=−59.249 Kcal/mol. For Pb—Te—O glass, the AG should be even smaller. PbO and Pb—Te—O solids have a smaller ΔG than tellurium oxide, which suggests lower reactivity with silicon. (Thermodynamic stability of binary oxides in contact with silicon, K. J. Hubbard and D. G. Schlom, J. Mater. Res., Vol. 11, No. 11, (1996)). It is believed that high reactivity with the silicon wafer could result in the formation of reactive contact points on the silicon wafer. The high reactivity of tellurium oxide may also contribute to formation of contact on certain high efficiency wafers with low surface doping concentration. Also, with tellurium oxide as an additive, the IRS system can be easily adapted to different glass frits and glass chemistries for a variety of electroconductive paste applications.

[0032] Incorporating tellurium oxide as an additive to the IRS also provides great flexibility to metallization paste formulation in industrial applications. Instead of making Pb—Te—O solids as glass frit and using such material in paste formulation, using tellurium oxide as an additive allows the paste reactivity to be easily adjusted to meet different Si-wafer (such as vary doping concentrations) and emitter structures.

[0033] The industrial trend for solar cell production is moving toward the fast firing process, with very fast belt speed and “Spike” firing profiles. The controllable high reactivity of the metallization paste according to the present invention with tellurium oxide additive is very suitable for such a process.

[0034] The tellurium oxide additive is between 5-95 wt.% of the IRS, more preferably between 10-60 wt. % of the IRS. In some embodiments, the tellurium oxide additive may comprise of submicron particles having a D50 less than 1 μM. In other embodiments, the tellurium oxide additive may be less than 10 μM in average particle size (D50).

[0035] The tellurium oxide additive is preferably tellurium dioxide (TeO.sub.2), although tellurium trioxide (TeO.sub.3) can also be used. In addition to tellurium oxides, other tellurium-oxygen compounds can be used, including, but not limited to, tellurious acid compound, telluric acid, organic telluric compounds, and any telluric compound that would produce telluride oxide during the firing process.

[0036] In a preferred embodiment, the lead containing composition is a type of glass frit (with an amorphous structure, and may also incorporate crystalline phases or compounds) having lead containing compounds as starting materials. The higher the amount of lead in the glass frit, the lower the glass transition temperature of the glass. However, higher lead amounts may also cause shunting in the semiconductor substrate, thereby decreasing the resulting solar cell's efficiency. In a preferred embodiment, lead oxide is used. More preferably, the glass frit contains about 35-95 wt. % lead oxide, preferably about 40-85 wt. % lead oxide.

[0037] The present invention IRS can have a PbO:Tellurium oxide additive weight percentage ratio of 95:5 to 5:95. Preferably, the PbO:Tellurium oxide additive weight percentage ratio is 10:1 to 1:10. More preferably, the PbO:Tellurium oxide additive weight percentage ratio is 5:1 to 1:5.

[0038] Glass frits of the present invention may also include other oxides or compounds known to one skilled in the art for making glass fits. For example, silicon, boron, aluminum, bismuth, lithium, sodium, magnesium, zinc, titanium, zirconium oxides and compounds. Other glass matrix formers or glass modifiers, such as germanium oxide, vanadium oxide, tungsten oxide, molybdenum oxides, niobium oxides, tin oxides, indium oxides, other alkaline and alkaline earth metal (such as K, Rb, Cs and Be, Ca, Sr, Ba) compounds, rare earth oxides (such as La.sub.2O.sub.3, cerium oxides), phosphorus oxides or metal phosphates, transition metal oxides (such as copper oxides and chromium oxides), metal halides (such as lead flurides and zinc flurides may also be part of the glass composition.

[0039] Lead containing glass frit can be made by any process known to one skilled in the art. For example, glass fit components, in powder form, may be mixed together in a V-comb blender. The mixture is then heated to a very high temperature (around 120° C.) for about 30-40 minutes. The glass is then quenched, taking on a sand-like consistency. This coarse glass powder is then milled, such as in a ball mill or jet mill, until a fine powder results. Lead containing glass frit can alternatively comprise lead oxides, 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. In another embodiment, lead oxide may be mixed directly with other components of the IRS of the present invention without the need of first processing the lead oxide into the form of a glass frit.

[0040] The IRS of the present invention may be created through any number of processes known to one skilled in the art. For example, the IRS particles, having an average particle size of around 0.1-10 μM (D50) are mixed from different raw IRS materials. The average particle size is dependent on the particle size of the raw IRS materials and the mixing process. A good mixing process should result in a well-dispersed mixture of the IRS components.

[0041] In another example, conventional solid state synthesis may be used to prepare the IRS. In this case, raw materials are sealed in a fused-quartz tube or tantalum or platinum tube under vacuum, and then heated to 700-1200° C. The materials dwell at this elevated temperature for 12-48 hours and then are slowly cooled (0.1° C./minute) to room temperature. In some cases, solid state reactions may be carried out in an alumina crucible in air.

[0042] In another example, co-precipitation may be used to form the IRS. In this process, the metal elements are reduced and co-precipitated with other metal oxides or hydroxides from a solution containing metal cations by adjusting the pH levels or by incorporating reducing agents. The precipitates of these metals, metal oxides or hydroxides are then dried and fired under vacuum at 400-600° C. and fine powders of the compounds are formed.

[0043] IRS according to the present invention may also comprise additional additives, which can be any oxides and compounds known to one skilled in the art to be useful as additives. For example, boron, aluminum, bismuth, lithium, sodium, magnesium, zinc, phosphate. Other glass matrix formers or glass modifiers, such as germanium oxide, vanadium oxide, tungsten oxide, molybdenum oxides, niobium oxides, tin oxides, indium oxides, other alkaline and alkaline earth metal (such as K, Rb, Cs and Be, Ca, Sr, Ba) compounds, rare earth oxides (such as La.sub.2O.sub.3, cerium oxides), phosphorus oxides or metal phosphates, transition metal oxides (such as copper oxides and chromium oxides), metal halides (such as lead flurides and zinc flurides) may also be used as additives to adjust glass properties such as glass transition temperature.

[0044] Another aspect of the present invention relates to an electroconductive paste composition which comprises metallic particles, the IRS of the present invention, and an organic vehicle. The preferred metallic particles are silver, but can be any known conductive metal or mixture thereof, including, but not limited to, gold, copper, or nickel. The metallic particles are about 50-95 wt. % of solid content of the paste, preferably about 75-95 wt. % of solid content of the paste. The IRS is about 1-10 wt. % of solid content of the paste, preferably about 2-8 wt. %, more preferably about 5% of solid content of the paste. The amount of tellurium oxide additive may also be measure based on weight percentage of paste. Typically the tellurium oxide additive can be of 0.1-5% wt. paste. More preferably, 0.3-5% wt. of paste.

[0045] The organic vehicle may comprise a binder and a solvent, as well as a surfactant and thixatropic agent. Typical compositions of the organic vehicle are known to those of skill in the art. For example, a common binder for such applications is a cellulose or phenolic resin, and common solvents can be any of carbitol, terpineol, hexyl carbitol, texanol, butyl carbitol, butyl carbitol acetate, or dimethyladipate or glycol ethers. The organic vehicle also includes surfactants and thixatropic agents known to one skilled in the art. Surfactants can include, but are not limited to, polyethyleneoxide, polyethyleneglycol, benzotriazole, poly(ethyleneglycol)acetic acid, lauric acid, oleic acid, capric acid, myristic acid, linolic acid, stearic acid, palmitic acid, stearate salts, palmitate salts, and mixtures thereof. The organic vehicle is about 1-20 wt. % of the paste, preferably about 5-15 wt. % of the paste. The thixatropic agent is about 0.1-5 wt. % of the paste.

[0046] To form an electroconductive paste, the IRS materials are combined with electroconductive particles, e.g., silver, and an 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 IRS materials can be co-milled with silver particles in a ball mill for 2-24 hours to achieve a homogenous mixture of IRS and silver particles, which are then combined with the organic solvent in a mixer.

[0047] Such a paste may then be utilized to form a solar cell by applying the paste to the antireflection layer on the silicon substrate, such as by screen printing, and then drying and firing to form an electrode on the silicon substrate.

[0048] The preferred embodiments as described above of the present invention IRS system with a lead containing matrix forming composition and a tellurium oxide additive and electroconductive paste made thereof are typically applied to the light receiving surface of a silicon wafer. Typically, the present invention electroconductive paste is screen printed over the ARC of a silicon wafer. Other application methods, such us stenciling, may also be used to apply the electroconductive paste. However, the foregoing does not preclude incorporating the present invention IRS system to an electroconductive paste intended for the backside of the silicon wafer.

EXAMPLE 1

[0049] As shown in Table 1, exemplary electroconductive pastes T1-T4 were prepared with an IRS comprising a glass frit comprising about 43% PbO (in IRS) and a number of metal oxide additives. Particularly, exemplary electroconductive paste T1 comprises 1.5% wt. paste of bismuth oxide (Bi.sub.2O.sub.3), T2 comprises 1.5% wt. paste of tellurium oxide (TeO.sub.2), T3 comprises 1.5% wt. paste of tin oxide (SnO), and T4 comprises 1.5% wt. paste of antimony trioxide (Sb.sub.2O.sub.3). Silver particles, in an amount of about 85 wt. % (of paste), and an organic vehicle, in an amount of about 1-10 wt. % (of paste), were added to form the exemplary pastes. Exemplary solar cells were prepared using lightly-doped p-type silicon wafers with a sheet resistance of 80 /□.

TABLE-US-00001 TABLE 1 Metal oxide additives in electroconductive paste formulations Refer- ence IRS Components paste T1 T2 T3 T4 Leaded Glass A 4.6 3.1 3.1 3.1 3.1 Glass Frit (43% PbO) Oxide Bi.sub.2O.sub.3 1.5 Additives TeO.sub.2 1.5 SnO 1.5 Sb.sub.2O.sub.3 1.5 Total 4.60 4.60 4.60 4.60 4.60 IRS % in paste PbO w. % 43.48% 43.48% 43.48% 43.48% 43.48% in IRS

[0050] The paste was screen printed onto the front side of silicon wafers at a speed of 150 mm/s, using a 325 (mesh)*0.9 (mil, wire diameter)*0.6 (mil, emulsion thickness)*70 μm (finger line opening) calendar screen. An aluminum back side paste was also applied to the back side of the silicon wafer. The printed wafer was dried at 150° C. and then fired at a profile with the peak temperature of about 750-900° C. for a few seconds in a linear multi-zone infrared furnace. A commercial paste is used as reference.

[0051] All solar cells were then tested using an I-V tester. A Xe arc lamp in the I-V tester was used to simulate sunlight with a known intensity and the front surface of the solar cell was irradiated to generate the I-V curve. Using this curve, various parameters common to this measurement method which provide for electrical performance comparison were determined, including solar cell efficiency (Eta %), series resistance under three standard lighting intensities (Rs3 mΩ), fill factor (FF %). Direct measurement of contact resistance by four-probe technique is always used by researchers, but the measurement accuracy is very dependent on sample preparation. Therefore, in the circumstance when the finger line resistivity (usually the same silver material and firing condition) and finger line geometry (printing related) are identical, series resistance Rs3 given by H.A.L.M IV tester can be used to evaluate the electrical contact behavior of the conductive paste to silicon substrate. Generally, the smaller the Rs3, the better contact behavior of the silver pastes. Data for the reference paste was normalized to 1. The relevant data for the experimental pastes was calculated by dividing the appropriate measurement by the normalized reference cell data. Selected electrical performance data for exemplary pastes T1-T4 is compiled in Table 2.

[0052] It is clear from the data presented in Table 2 that exemplary paste T2 with tellurium oxide additive unexpectedly improves serial resistance, as shown through significant reduction of the Rs3 measurement, which also results in increased solar cell efficiency and fill factor gain. Other tested metal oxides failed to show similar beneficial effects, even though these metal oxides are also known to modify glass softening temperature and adjust glass flowability.

TABLE-US-00002 TABLE 2 Electric performance of solar cell produced using electroconductive paste formulations comprising metal oxide additives Reference paste T1 T2 T3 T4 Eta 1.0000 1.0006 1.0077 1.0036 0.8349 FF 1.0000 1.0052 1.0081 1.0040 0.8366 Rs3 1.0000 0.9771 0.9462 1.0548 4.9133

EXAMPLE 2

[0053] A number of additional exemplary pastes E1-E22 were prepared with four glass frits A (43% PbO), B (60% PbO), C (67% PbO), D (73% PbO) and various amounts of tellurium oxide additive. Details of the exemplary paste formulations and PBO and TeO.sub.2 content and weight ratio of each exemplary paste is represented in Table 3. Exemplary solar cells, using mono-crystalline or polycrystalline silicon wafers with varying sheet resistance, were prepared by the process set forth in Example 1 above. More specifically, selected electrical performance data of solar cells prepared with multi-crystalline silicon wafer type 1A (sheet resistance of 70Ω/□) is present in Table 4, electrical performance data of solar cells prepared with multi-crystalline silicon wafer 1B (sheet resistance of 95Ω/□ is present in Table 5, electrical performance data of solar cells prepared with mono-crystalline silicon wafer type 2 (sheet resistance of 65Ω/□ is present in Table 6, electrical performance data of solar cells prepared with mono-crystalline silicon wafer type 3 (sheet resistance of 90Ω/□) is present in Table 7, electrical performance data of solar cells prepared with multi-crystalline silicon wafer type 4 (sheet resistance of 60Ω/□ is present in Table 8, electrical performance data of solar cells prepared with mono-crystalline silicon wafer type 5 (sheet resistance of 60Ω/□ is present in Table 9, electrical performance data of solar cells prepared with multi-crystalline silicon wafer type 6 (sheet resistance of 70Ω/□ is present in Table 10, electrical performance data of solar cells prepared with mono-crystalline silicon wafer type 7 (sheet resistance of 70Ω/□ is present in Table 11, and electrical performance data of solar cells prepared with multi-crystalline silicon wafer type 8 (sheet resistance of 65Ω/□ is present in Table 12. All data are normalized against reference paste on silicon wafer type 1A. Particularly, relative efficiency, relative fill factor, and relative Rs3 measurements for all exemplary pastes E1-E22 on all other silicon wafer types are normalized against efficiency, fill factor, and Rs3 measurements of reference paste on silicon wafer type 1A, respectively.

TABLE-US-00003 TABLE 3 IRS formulations with tellurium oxide additive (Component wt. % in Paste) Refer- IRS IRS ence Components Components paste E1 E2 E3 E4 E5 E6 E7 E8 E9 E10 Leaded Glass A 4.6 3.1 2.9 2.7 Glass (43% PbO) Frit Glass B 2.09 (60% PbO) Glass C 3.1 3.1 3.1 2.79 2.48 2.79 (67% PbO) Glass D (73% PbO) TeO.sub.2 1.5 1.5 1.5 2.51 1.5 1.2 1 0.7 0.8 0.9 (wt. % paste) Total IRS 4.6 4.6 4.4 4.2 4.6 4.6 4.3 4.1 3.49 3.28 3.69 (wt. % paste) PbO 43.48% 43.73% 45.72% 47.89% 26.88% 45.00% 48.14% 50.49% 53.38% 50.49% 50.49% w. % in IRS TeO.sub.2 — 32.61% 34.09% 35.71% 54.57% 32.61% 27.91% 24.39% 20.06% 24.39% 24.39% w. % in IRS PbO/TeO.sub.2 — 0.90 0.88 0.86 0.22 0.93 1.24 1.57 2.13 1.57 1.57 ratio, weight IRS IRS Components Components E11 E12 E13 E14 E15 E16 E17 E18 E19 E20 E21 E22 Leaded Glass A Glass (43% PbO) Frit Glass B (60% PbO) Glass C 2.79 3.1 2.66 2.48 2.61 2.03 2.17 1.96 (67% PbO) Glass D 3.2 2.7 2.2 1.7 (73% PbO) TeO.sub.2 0.6 0.5 0.9 0.6 0.9 0.5 0.7 0.7 1 1 1 1 (wt. % paste) Total IRS 3.39 3.6 3.555 3.08 3.51 2.53 2.87 2.66 4.2 3.7 3.2 2.7 (wt. % paste) PbO 54.96% 57.50% 52.41% 53.77% 53.08% 57.27% 50.49% 54.47% 55.24% 50.16% 43.50% 34.37% (wt. % in IRS) TeO.sub.2 17.70% 13.89% 25.32% 19.48% 25.64% 19.76% 24.39% 26.32% 23.81% 27.03% 31.25% 37.04% (wt. % in IRS) PbO/TeO.sub.2 2.56 3.57 1.55 2.22 1.54 2.33 1.57 1.53 1.77 1.35 0.96 0.58 ratio, weight

TABLE-US-00004 TABLE 4 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 1A Reference paste E1 E19 E20 E21 E22 Eta 1.0000 1.0056 1.0134 1.0202 1.0212 1.0111 FF 1.0000 1.0131 1.0144 1.0137 1.0189 1.0132 Rs3 1.0000 0.7963 0.6804 0.6748 0.8198 0.8076

TABLE-US-00005 TABLE 5 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 1B Reference paste E16 E17 Eta 1.0053 1.0201 1.0272 FF 0.9808 0.9973 1.0032 Rs3 1.4524 0.8204 0.7654

TABLE-US-00006 TABLE 6 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 2 Reference paste E1 E2 E3 E4 Eta 1.0378 1.0809 1.0856 1.0773 1.0773 FF 0.9912 1.0242 1.0279 1.0119 0.9901 Rs3 0.9526 0.4839 0.4938 0.6154 0.8897

TABLE-US-00007 TABLE 7 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 3 Reference paste E1 E2 Eta 0.8040 1.1063 0.9976 FF 0.7360 1.0130 0.9105 Rs3 4.1267 0.6909 1.9430

TABLE-US-00008 TABLE 8 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 4 Refer- ence paste E10 E11 E12 E13 E14 E15 Eta 0.9787 0.9876 0.9900 0.9911 0.9841 0.9900 0.9876 FF 0.9926 1.0097 1.0091 0.9966 1.0087 1.0083 1.0099 Rs3 0.7915 0.6491 0.6647 0.8200 0.6847 0.6676 0.6636

TABLE-US-00009 TABLE 9 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 5 Refer- ence paste E5 E6 E7 E8 E10 E15 Eta 0.9669 0.9805 0.9876 0.9876 0.9852 0.9935 0.9935 FF 0.9917 1.0097 1.0105 1.0117 1.0092 1.0097 1.0099 Rs3 1.0455 0.6733 0.6488 0.6597 0.6985 0.6491 0.6636

TABLE-US-00010 TABLE 10 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 6 Reference paste E5 E6 E7 E8 E9 Eta 0.9693 0.9829 0.9882 0.9947 0.9953 0.9811 FF 0.9900 1.0157 1.0154 1.0156 1.0174 1.0140 Rs3 1.1562 0.5718 0.5524 0.5713 0.5595 0.5728

TABLE-US-00011 TABLE 11 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 7 Reference paste E16 E17 E18 Eta 1.0915 1.1175 1.1163 1.1151 FF 1.0144 1.0329 1.0345 1.0333 Rs3 0.7726 0.5379 0.5366 0.5337

TABLE-US-00012 TABLE 12 Electric performance of solar cell produced using electroconductive paste formulations comprising tellurium oxide additive on silicon wafer type 8 Reference paste E11 E16 Eta 0.9658 0.9752 0.9929 FF 0.9689 0.9946 0.9987 Rs3 1.1484 0.8387 0.9133

[0054] As shown in Tables 4-12, exemplary pastes E1-E22 are shown to produce solar cells with overall improved serial resistance as evidenced by the Rs3 measurements. The improvement in serial resistance also contributes to improved overall solar cell performance. For all types of silicon wafers tested, the exemplary pastes outperform the reference paste in terms of relative efficiency and/or fill factor. The most dramatic improvement over the commercial reference paste is shown through exemplary pastes comprising tellurium oxide additives according to the present invention with high sheet resistance silicon wafers, e.g., type 3 wafer (mono-crystalline with 90Ω/□ of sheet resistance). The reference paste performed poorly with this type of silicon wafer, providing poor serial resistance and overall subpar solar cell performance. The exemplary pastes E1 and E2, comprising the same type of PbO containing glass frit as the reference paste, showed drastically improved performance over the reference paste, providing very good serial resistance measurements and superior solar cell overall performance.

[0055] The electrical performance data for the reference paste shown in Table 4-12 also clearly demonstrates one persisting difficulty for the metallization paste technology. The same reference paste, applied using exactly the same screen printing and firing procedure on a number of silicon wafers of varying sheet resistance, produced solar cells of disparaging performance characteristics. For example, the relevant efficiency of the cells for the reference paste in Table 4-12 is from 0.8040 to 1.0915, which is a difference of over 28%. As an industrial process, such large variance is not acceptable. Paste composition thus must to be modified for each type of silicon wafer to achieve optimal performance of the resulting solar cells. The modification process is typically time-consuming, since an electroconductive paste comprises a number of components, allow of which may need to be optimized.

[0056] The present invention incorporates a tellurium oxide additive. As an additive, and not part of the matrix forming composition, the amount of the tellurium oxide additive can be readily adjusted for different types of silicon wafer. It is believed that reactivity of the IRS system can be fine-tuned by adjusting the tellurium oxide additive. The data presented in Table 4-12 shows that by adjusting lead containing matrix forming compositions and tellurium oxide additive, optimal performance can be expected from all the tested silicon wafers. For exemplary pastes E1-E22, types and amounts of lead containing glass frit and amounts of tellurium oxide are adjusted. As shown above, the resulting solar cells outperform the reference paste in terms of relative efficiency and/or fill factor (Table 4-12). This is particularly true for silicon wafers where the reference paste fails to perform (Table 7).

[0057] These and other advantages of the present 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.