GLASS FRIT, CONDUCTIVE PASTE AND USE OF THE CONDUCTIVE PASTE

Abstract

The invention relates to a glass frit being a mixture of a first glass frit comprising tellurium oxide and bismuth oxide as main components and a second glass frit comprising tellurium oxide and lead oxide as main components, wherein the mixture of the first glass frit and the second glass frit comprises 40 to 55% by weight of tellurium oxide, 15 to 25% by weight of lead oxide and 5 to 15% by weight of bismuth oxide. The invention further relates to a conductive paste for forming electrodes on a semiconductor substrate, the paste comprising 85 to 92% by weight of an electrically conductive metal, 1.5 to 3.5% by weight of the glass frit and organic medium. The conductive paste is used for forming electrically conductive grid lines on semiconductor substrates for solar cells.

Claims

1: A glass frit, comprising a mixture of a first glass frit comprising tellurium oxide and bismuth oxide as main components and a second glass frit comprising tellurium oxide and lead oxide as main components, wherein the mixture of the first glass frit and the second glass frit comprises 40 to 55% by weight of tellurium oxide, 15 to 25% by weight of lead oxide and 5 to 15% by weight of bismuth oxide.

2: The glass frit of claim 1, wherein the first glass frit comprises 40 to 70% by weight of TeO.sub.2 and 0.1 to 15% by weight of Bi.sub.2O.sub.3.

3: The glass frit of claim 2, wherein the first glass frit further comprises 0.1 to 15% by weight of SiO.sub.2, 0.1 to 15% by weight of ZnO, 0.1 to 15% by weight of WO.sub.3 and 0 to 10% by weight of Li.sub.2O.

4: The glass frit of claim 3, wherein the first glass frit additionally comprises one or more of Cs.sub.2O.sub.3, MgO, V.sub.2O.sub.5, ZrO.sub.2, Mn.sub.2O.sub.3, Ag.sub.2O, In.sub.2O.sub.3, SnO.sub.2, NiO, Cr.sub.2O.sub.3, B.sub.2O.sub.3, Na.sub.2O, Al.sub.2O.sub.3 and CaO, each in an amount in a range of from 0 to 10% by weight.

5: The glass frit of claim 1, wherein the second glass frit comprises 40 to 70% by weight of TeO.sub.2 and 5 to 30% by weight of PbO.

6: The glass frit of claim 5, wherein the second glass frit further comprises 0.1 to 15% by weight of Bi.sub.2O.sub.3, 0.1 to 15% by weight of SiO.sub.2, 0.1 to 10% by weight of ZnO, 0.1 to 10% by weight of WO.sub.3 and 0.1 to 10% by weight of Li.sub.2O.

7: The glass frit of claim 6, wherein the second glass frit additionally comprises one or more of Cs.sub.2O.sub.3, MgO, V.sub.2O.sub.5, ZrO.sub.2, Mn.sub.2O.sub.3, Ag.sub.2O, In.sub.2O.sub.3, SnO.sub.2, NiO, Cr.sub.2O.sub.3, B.sub.2O.sub.3, Na.sub.2O, Al.sub.2O.sub.3 and CaO, each in an amount in a range of from 0 to 10% by weight.

8: A conductive paste, comprising: (a) 85 to 92% by weight of an electrically conductive metal, (b) 1.5 to 3.5% by weight of the glass frit of claim 1, and (c) an organic medium.

9: The conductive paste of claim 8, wherein the electrically conductive metal is selected from the group consisting of carbon, silver, gold, aluminum, platinum, palladium, tin, nickel, cadmium, gallium, indium, copper, zinc, iron, bismuth, cobalt, manganese, molybdenum, chromium, vanadium, titanium, tungsten, alloys thereof and mixtures thereof.

10: The conductive paste of claim 8, wherein the organic medium is selected from the group consisting of solvents, binders, surfactants, thixotropic agents, plasticizers, solubilizers, defoamers, desiccants, crosslinkers, complexing agents and/or conductive polymer particles and mixtures thereof.

11: The conductive paste of claim 8, wherein the electrically conductive metal is in the form of particles having a mean particle size in a range of from 10 nm to 100 m.

12: A semiconductor substrate, comprising electrically conductive grid lines formed on the semiconductor substrate by the conductive paste of claim 8.

Description

EXAMPLES

[0045] A conductive paste has been prepared by mixing 90% by weight silver powder having a mean particle size of 3% by weight of glass frit and 7% by weight organic medium. The composition of the first and the second glass frit is shown in table 1.

TABLE-US-00001 TABLE 1 Composition of the glass frit component Mixture #1 Mixture #2 Mixture #3 TeO.sub.2 65 55 50 PbO 5 20 25 Bi.sub.2O.sub.3 10 11 12 SiO.sub.2 7 6 5 ZnO 4 2 1

[0046] The pastes were applied to 6 multi-crystalline (Table 2) and mono-crystalline (Table 3) wafers and with a sheet resistance of 80/ phosphorous-doped emitter on a p-type base. The solar cells used were textured by isotropic acid etching and had an 80 nm anti-reflection coating (ARC) of SiNX:H. For each paste, the mean values of the efficiency and fill factor for 15 pieces of silicon wafer are shown. Each sample was made by screen-printing using a micro-tec MT650 printer set with a squeegee speed of 250 mm/sec. The screen used had a pattern of 105 finger lines with a 32 m opening and 4 bus bar with a 1.0 mm opening on a 14 m emulsion in a screen with 360 mesh and 16 m wires. A commercially available Al paste was printed on the non-illuminated (back) side of the device. The Al paste was printed with 5 m emulsion in a screen with 250 mesh and 35 m wires.

[0047] The device with the printed patterns was then dried in a drying oven with a 250 C. peak temperature. The substrates were then fired sun-side up with a CF-SL Despatch 6-zone IR furnace using a 635 cm/min belt speed and 920 C. (as shown in Table 2), and 900 C., 910 C and 920 C. (as shown in Table 3), as setting temperature of the 6.sup.th zone in the furnace.

[0048] The solar cells built according to the method described herein were tested for conversion efficiency.

[0049] In an embodiment, the solar cells built according to the method described herein were placed in a commercial I-V tester for measuring efficiencies (halm gmbh, cetisPV-Celltest3). The Xe Arc lamp in the I-V tester simulated the sunlight with a known intensity, AM 1.5, and irradiated the front surface of the cell. The tester used a four-contact method to measure current (I) and voltage (V). Solar cell efficiency (Eta), open-circuit voltage (Voc) and fill factor (FF) were calculated from the I-V curve.

[0050] As can be seen from table 2, the inventive paste shows noticeably higher solar cell efficiency as comparable to a reference paste. Table 3 further shows the inventive paste gives promising solar cell efficiency without loss of fill factor over the firing temperature range.

TABLE-US-00002 TABLE 2 Voc (open-circuit voltage), Solar cell efficiency and fill factor compared to a reference paste Setting Paste temperature ( C.) Voc (mV) FF (%) Eta (%) Reference paste 920 629.6 79.7 18.0 Mixture #1 920 630.0 79.4 18.1 Mixture #2 920 630.6 79.7 18.3 Mixture #3 920 631.0 79.7 18.2

TABLE-US-00003 TABLE 3 Voc (open-circuit voltage), Solar cell efficiency and fill factor over firing range. Setting Mixture temperature ( C.) Voc (mV) FF (%) Eta (%) Mixture #2 900 643.0 80.5 19.7 Mixture #2 910 643.7 80.6 19.8 Mixture #2 920 642.2 80.6 19.7