Conductive paste composition and method for manufacturing electrode

Abstract

Disclosed is a conductive paste composition, including 100 parts by weight of copper powder, 40 to 150 parts by weight of silver powder, 0.1 to 3 parts by weight of carbon powder, 1 to 5 parts by weight of glass powder, and 5 to 15 parts by weight of binder. The conductive paste composition can be applied on a substrate, and then sintered under atmosphere at a high temperature to form an electrode on the substrate.

Claims

1. An unsintered conductive paste composition, comprising: 100 parts by weight of uncoated copper powder; 40 to 150 parts by weight of silver powder; 0.1 to 3 parts by weight of carbon powder; 1 to 5 parts by weight of glass powder; and 5 to 15 parts by weight of binder, wherein the uncoated copper powder has a particle size greater than that of the silver powder, and wherein the carbon powder is carbon nanotube or graphene with a specific surface area of 200 m.sup.2/g to 1000 m.sup.2/g.

2. The unsintered conductive paste composition as claimed in claim 1, wherein the uncoated copper powder has a particle size of 1 μm to 10 μm.

3. The unsintered conductive paste composition as claimed in claim 1, wherein the silver power has a particle size of 0.1 μm to 2 μm.

4. The unsintered conductive paste composition as claimed in claim 1, wherein the binder comprises epoxy resin.

5. A method for manufacturing an electrode, comprising: applying the unsintered conductive paste composition as claimed in claim 1 onto a substrate; and sintering the unsintered conductive paste composition under a normal atmosphere to form an electrode on the substrate.

6. The method as claimed in claim 5, wherein the unsintered conductive paste composition is sintered at a temperature of 600° C. to 900° C.

Description

DETAILED DESCRIPTION

(1) In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details.

(2) The conductive paste composition of the disclosure includes 100 parts by weight of copper powder, about 40 to 150 parts by weight of silver powder, about 0.1 to 3 parts by weight of carbon powder, about 1 to 5 parts by weight of glass powder, and about 5 to 15 parts by weight of binder. When the conductive paste is sintered at a high temperature, the carbon powder in the conductive paste composition may inhibit the oxidation of the copper and chemically reduce the generation of copper oxide. As such, an electrode with low resistivity is formed.

(3) In one embodiment of the disclosure, the copper powder has a particle size of 1 μm to 10 μm. A copper powder having an overly large particle size may block the screen during printing and lower the delicate degree of the printed pattern. The copper powder having an overly small particle size will be easily oxidized due to its high specific surface area. In addition, the copper powder has a particle size larger than that of the silver powder. If the silver powder has a particle size that is equal to or greater than that of the copper powder, the silver powder cannot efficiently wrap the surface of the copper powder, thereby failing to insulate oxidation of the copper powder.

(4) In one embodiment of the disclosure, the silver powder has a particle size of about 0.1 μm to 2 μm. A silver powder having an overly large particle size cannot efficiently wrap the surface of the copper powder. A silver powder having an overly small particle size is easily aggregated, and is difficult to be dispersed. In addition, an overly high amount of the silver powder will greatly increase the cost of the conductive paste. An overly low amount of the silver powder cannot form an electrode with sufficient conductivity by sintering the conductive paste at a high temperature.

(5) In one embodiment of the disclosure, the carbon powder can be graphene, active carbon, carbon nanotube, artificial graphite, or natural graphite. In one embodiment, the carbon powder has a specific surface area of about 200 m.sup.2/g to 1000 m.sup.2/g. A carbon powder having an overly low specific surface area cannot efficiently inhibit the oxidation of copper during the sintering of the conductive paste at a high temperature. In addition, an overly high amount of carbon powder will reduce the conductivity of the electrode formed by sintering the conductive paste. An overly low amount of carbon powder cannot efficiently inhibit the oxidation of copper during the sintering of the conductive paste at a high temperature.

(6) In one embodiment, the glass powder has a softening point that is lower than or equal to 600° C. and a crystallization onset temperature that is higher than 600° C. The glass powder can be BBG-3 or VBP commercially available from Okamoto Glass Co., Ltd.

(7) In one embodiment of the disclosure, the binder includes epoxy resin and hardener. For example, the epoxy resin can be bisphenol epoxy resin, o-cresol novolac epoxy resin, cycloaliphatic epoxy resin, or another suitable epoxy resin. For example, the bisphenol epoxy resin can be EPON 828 commercially available from Shell Company, H-4032D commercially available from DIC Company, or EXA-830LVP commercially from DIC Company. The o-cresol novolac epoxy resin can be CNE 202 commercially available from Chang Chun Chemical. The cycloaliphatic epoxy resin can be ERL 4221E or ERL 4206 commercially available from U.C. company, EPALLOY™ 5200 commercially available from CVC company, or EPALLOY™ 5001 commercially available from CVC company. Other epoxy resins may be GS120 commercially available from CVC Company, or GE20 commercially available from CVC Company. The hardener can be amine hardener, acid anhydride hardener, cationic hardener, or combinations thereof. The amine hardener can be D230, D400, or D2000 commercially available from Huntsman Company, or combinations thereof. The acid anhydride hardener can be MHHPA-MW, MTHPA, or NMA commercially available from Lonza Company, or combinations thereof. The cationic hardener can be SI-45, SI-60, SI-B2A, SI-B3, or SI-B3A commercially available from Sanshin Chemical Industry co., Ltd, or combinations thereof. In one embodiment, the binder may further include reaction promoter to reduce the curing temperature of the epoxy resin. The reaction promoter can be SA102 commercially available from SAN-APRO, 2E4MZ commercially available from TCI, or TMAH commercially available from Aldrich.

(8) In another embodiment, solvent, thickener, dispersant, thixotropic agent, defoamer, or combinations thereof can be added into the conductive paste composition to improve the processability of the conductive paste.

(9) In one embodiment, the conductive paste is applied onto a substrate, and then sintered under a normal atmosphere at a high temperature to form an electrode on the substrate. In one embodiment, the substrate should resist the high temperature sintering, like a ceramic substrate or silicon wafer. The high temperature required for sintering is about 500° C. to 900° C., about 600° C. to 900° C., or about 700° C. to 900° C. After sintering the conductive paste at a high temperature, an electrode with high conductivity and low resistance is formed, for application as the surface electrode of a solar cell (or other devices manufactured by a high-temperature process). Because the carbon powder may inhibit the oxidation of copper and chemically reduce the generation of copper oxide, the high temperature sintering can be performed under a normal atmosphere, rather than under an inert gas or a chemically reducing atmosphere. As such, the sintering step is simplified.

(10) Below, exemplary embodiments will be described in detail so as to be easily realized by a person having ordinary knowledge in the art. The inventive concept may be embodied in various forms without being limited to the exemplary embodiments set forth herein. Descriptions of well-known parts are omitted for clarity, and like reference numerals refer to like elements throughout.

EXAMPLES

Example 1

(11) 16.76 g of EPON 828, 52.6 g of ERL 4221E, 57.92 g of MHHPA-MW, and 0.694 g of tetramethylammonium hydroxide (TMAH) were mixed to form a binder A. 70 g of copper powder (mean particle size of 10 μm, commercially available from DOWA), 30 g of silver powder (mean particle size of 2.0 μm, commercially available from Ferro), 5 g of glass powder (mean particle size of 3.0 μm, commercially available from Okamoto Glass Co., Ltd.), and 0.1 g of graphene (specific surface area of about 500 m.sup.2/g, prepared according to H. A. Becerill, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and Y. Chen, ACS Nano 2, 463 (2008)) were mixed with 15 g of the binder A by high-speed stiffing. The mixture was then milled and dispersed to form a conductive paste. The conductive paste was screen printed on a polysilicon substrate, and then sintered in an oven at 800° C. for 30 minutes to form an electrode on the substrate. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is shown in Table 1.

Example 2

(12) Example 2 was similar to Example 1. The difference between Examples 1 and 2 was the graphene amount being increased to 0.5 g in Example 2. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 1. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 1. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 1.

Example 3

(13) Example 3 was similar to Example 1. The differences between Examples 1 and 3 were the copper powder in Example 3 being changed to another copper powder (particle size of 4.7 μm, Cu-HWQ commercially available from Fukuda), the silver powder in Example 3 being changed to another silver powder (particle size of 0.8 μm, AG-2-1C commercially available from DOWA), and the graphene in Example 1 being replaced by a carbon nanotube (specific surface area of about 200 m.sup.2/g, MWCNT commercially available from Seedchem Company Pty., Ltd.) in Example 3. The amounts of the copper powder, the silver powder, the glass powder, and the binder were similar to those in Example 1. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 1. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 1.

Example 4

(14) Example 4 was similar to Example 1. The differences between Examples 1 and 4 were the copper powder in Example 4 being changed to another copper powder (particle size of 4.7 μm, Cu-HWQ commercially available from Fukuda), the silver powder in Example 4 being changed to another silver powder (particle size of 0.8 μm, AG-2-1C commercially available from DOWA), and 0.1 g of the graphene in Example 1 being replaced by 0.5 g of carbon nanotube (specific surface area of about 200 m.sup.2/g, MWCNT commercially available from Seedchem Company Pty., Ltd.) in Example 4. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 1. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 1. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 1.

Comparative Example 1

(15) Comparative Example 1 was similar to Example 1. The difference between Example 1 and Comparative Example 1 was the graphene in Example 1 being omitted in Comparative Example 1. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 1. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 1. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 1.

Comparative Example 2

(16) Comparative Example 2 was similar to Example 1. The difference between Examples 1 and Comparative Example 2 was the graphene in Example 1 being replaced by graphite (specific surface area of about 20 m.sup.2/g, KS-6 commercially available from TIMCAL graphite) in Comparative Example 2. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 1. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 1. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 1.

Comparative Example 3

(17) Comparative Example 3 was similar to Example 1. The difference between Examples 1 and Comparative Example 3 was 0.1 g of the graphene in Example 1 being replaced by 0.5 g of the graphite (specific surface area of about 20 m.sup.2/g, KS-6 commercially available from TIMCAL graphite) in Comparative Example 3. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 1. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 1. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 1.

(18) TABLE-US-00001 TABLE 1 Silver Copper Additive ratio (on Volume powder powder the basis of the Glass resistivity (g) (g) Additive copper powder) powder (g) Binder (g) (μΩ-cm) Example 1 30 70 Graphene 0.14% 5 15 297 (0.1 g) Example 2 30 70 Graphene 0.71% 5 15 88 (0.5 g) Example 3 30 70 Carbon 0.14% 5 15 222 nanotube (0.1 g) Example 4 30 70 Carbon 0.71% 5 15 107 nanotube (0.5 g) Comparative 30 70 None   0% 5 15 460 Example 1 Comparative 30 70 Graphite 0.14% 5 15 523 Example 2 (0.1 g) Comparative 30 70 Graphite 0.71% 5 15 445 Example 3 (0.5 g)

(19) As shown in Table 1, not only the graphene but also the carbon nanotube (with a high specific surface area) could reduce the volume resistivity of the electrode made from the conductive pastes. Furthermore, not all carbon material could be utilized to reduce the volume resistivity of the electrode made from the conductive pastes. For example, the graphite with low specific surface area could not reduce the volume resistivity of the electrode made from the conductive pastes.

Example 5

(20) 20 g of EPALLOY™ 5200, 20 g of ERL 4221E, and 1.2 g of SI-60 were mixed to form a binder B. 50 g of copper powder (mean particle size of 10 μm, commercially available from DOWA), 50 g of silver powder (mean particle size of 2.0 μm, commercially available from Ferro), 5 g of glass powder (mean particle size of 3.0 μm, commercially available from Okamoto Glass Co., Ltd.), and 0.1 g of graphene (specific surface area of about 500 m.sup.2/g, prepared according to H. A. Becerill, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and Y. Chen, ACS Nano 2, 463 (2008)) were mixed with 15 g of the binder B by high-speed stirring. The mixture was then milled and dispersed to form a conductive paste. The conductive paste was screen printed on a polysilicon substrate, and then sintered in an oven at 800° C. for 30 minutes to form an electrode on the substrate. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 2.

Example 6

(21) Example 6 was similar to Example 5. The difference between Examples 5 and 6 was the graphene amount being increased to 0.2 g in Example 6. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 5. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 5. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 2.

Comparative Example 4

(22) Comparative Example 4 was similar to Example 5. The difference between Example 5 and Comparative Example 4 was the graphene in Example 5 being omitted in Comparative Example 4. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 5. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 5. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 2.

(23) TABLE-US-00002 TABLE 2 Additive ratio (on the Silver Copper basis of the Volume powder powder copper Glass resistivity (g) (g) Additive powder) powder (g) Binder (g) (μΩ-cm) Example 5 50 50 Graphene 0.20% 5 15 111 (0.1 g) Example 6 50 50 Graphene 0.40% 5 15 92 (0.2 g) Comparative 50 50 — — 5 15 374 Example 4

(24) As shown in Table 2, graphene may reduce the volume resistivity of the electrode made from the conductive pastes.

Example 7

(25) 40 g of copper powder (mean particle size of 10 μm, commercially available from DOWA), 60 g of silver powder (mean particle size of 2.0 μm, commercially available from Ferro), 5 g of glass powder (mean particle size of 3.0 μm, commercially available from Okamoto Glass Co., Ltd.), and 0.1 g of graphene (specific surface area of about 500 m.sup.2/g, prepared according to H. A. Becerill, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and Y. Chen, ACS Nano 2, 463 (2008)) were mixed with 15 g of the binder A in Example 1 by high-speed stirring. The mixture was then milled and dispersed to form a conductive paste. The conductive paste was screen printed on a polysilicon substrate, and then sintered in an oven at 800° C. for 30 minutes to form an electrode on the substrate. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Example 8

(26) Example 8 was similar to Example 7. The difference between Examples 7 and 8 was the graphene amount being increased to 0.2 g in Example 8. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Example 9

(27) Example 9 was similar to Example 7. The differences between Examples 7 and 9 were the copper powder in Example 9 being changed to another copper powder (particle size of 4.7 μm, Cu-HWQ commercially available from Fukuda), the silver powder in Example 9 being changed to another silver powder (particle size of 0.8 μm, AG-2-1C commercially available from DOWA), and 0.1 g of the graphene in Example 7 being replaced by 0.5 g of carbon nanotube (specific surface area of about 200 m.sup.2/g, MWCNT commercially available from Seedchem Company Pty., Ltd.) in Example 9. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Example 10

(28) Example 10 was similar to Example 7. The differences between Examples 7 and 10 were the copper powder in Example 10 being changed to another copper powder (particle size of 4.7 μm, Cu-HWQ commercially available from Fukuda), the silver powder in Example 10 being changed to another silver powder (particle size of 0.8 μm, AG-2-1C commercially available from DOWA), and 0.1 g of the graphene in Example 7 being replaced by 0.2 g of carbon nanotube (specific surface area of about 200 m.sup.2/g, MWCNT commercially available from Seedchem Company Pty., Ltd.) in Example 10. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Comparative Example 5

(29) Comparative Example 5 was similar to Example 7. The difference between Example 7 and Comparative Example 5 was the graphene in Example 7 being omitted in Comparative Example 5. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Comparative Example 6

(30) Comparative Example 6 was similar to Example 7. The difference between Example 7 and Comparative Example 6 was the 0.1 g of the graphene in Example 7 being replaced by 3 g of nickel powder (commercially available from Force Applied Material Tech Co.) in Comparative Example 6. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Comparative Example 7

(31) Comparative Example 7 was similar to Example 7. The difference between Example 7 and Comparative Example 7 was the 0.1 g of the graphene in Example 7 being replaced by 3 g of aluminum powder (commercially available from Force Applied Material Tech Co.) in Comparative Example 7. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Comparative Example 8

(32) Comparative Example 8 was similar to Example 7. The difference between Example 7 and Comparative Example 8 was the 0.1 g of the graphene in Example 7 being replaced by 0.1 g of graphite (specific surface area of about 20 m.sup.2/g, KS-6 commercially available from TIMCAL graphite) in Comparative Example 8. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Comparative Example 9

(33) Comparative Example 9 was similar to Example 7. The difference between Example 7 and Comparative Example 9 was the 0.1 g of the graphene in Example 7 being replaced by 0.5 g of graphite (specific surface area of about 20 m.sup.2/g, KS-6 commercially available from TIMCAL graphite) in Comparative Example 9. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

Comparative Example 10

(34) Comparative Example 10 was similar to Example 7. The difference between Example 7 and Comparative Example 10 was the 0.1 g of the graphene in Example 7 being replaced by 10 g of graphite (specific surface area of about 20 m.sup.2/g, KS-6 commercially available from TIMCAL graphite) in Comparative Example 8. The amounts of the copper power, the silver powder, the glass powder, and the binder were similar to those in Example 7. The steps of mixing, milling and dispersing, printing, and sintering to form the electrode were similar to those in Example 7. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 3.

(35) TABLE-US-00003 TABLE 3 Additive ratio (on the Silver Copper basis of the Volume powder powder copper Glass Binder resistivity (g) (g) Additive powder) powder (g) (g) (μΩ-cm) Example 7 60 40 Graphene 0.25% 5 15 52 (0.1 g) Example 8 60 40 Graphene 0.50% 5 15 35 (0.2 g) Example 9 60 40 CNT (0.1 g) 0.25% 5 15 50 Example 10 60 40 CNT (0.2 g) 0.50% 5 15 41 Comparative 60 40 — — 5 15 96 Example 5 Comparative 60 40 Ni (3 g) 7.50% 5 15 94 Example 6 Comparative 60 40 Al (3 g) 7.50% 5 15 124 Example 7 Comparative 60 40 KS-6 (0.1) 0.25% 5 15 91 Example 8 Comparative 60 40 KS-6 (0.5) 0.50% 5 15 90 Example 9 Comparative 60 40  KS-6 (10.0) 2.50% 5 15 306 Example 10

(36) As shown in Table 3, not only the graphene but also the carbon nanotube with a high specific surface area could reduce the volume resistivity of the electrode made from the conductive paste. In addition, not all carbon material could be utilized to reduce the volume resistivity of the electrode made from conductive paste. Furthermore, the metal powder, such as nickel powder or aluminum powder, could not reduce the volume resistivity of the electrode made from the conductive paste.

Comparative Example 11

(37) 40 g of copper powder (mean particle size of 0.1 μm, prepared according to H. T. Zhu et al., Journal of Crystal Growth, 270, 722 (2004)), 60 g of silver powder (mean particle size of 2.0 μm, commercially available from Ferro), and 5 g of glass powder (mean particle size of 3.0 μm, commercially available from Okamoto Glass Co., Ltd.) were mixed with 15 g of the binder A in Example 1 by high-speed stiffing. The mixture was then milled and dispersed to form a conductive paste. The conductive paste was screen printed on a polysilicon substrate, and then sintered in an oven at 800° C. for 30 minutes to form an electrode on the substrate. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 4.

Comparative Example 12

(38) 40 g of copper powder (mean particle size of 0.1 μm, prepared according to H. T. Zhu et al., Journal of Crystal Growth, 270, 722 (2004)), 60 g of silver powder (mean particle size of 2.0 μm, commercially available from Ferro), 5 g of glass powder (mean particle size of 3.0 μm, commercially available from Okamoto Glass Co., Ltd.), and 0.5 g of carbon nanotube (specific surface area of about 200 m.sup.2/g, MWCNT commercially available from Seedchem Company Pty., Ltd.) were mixed with 15 g of the binder A in Example 1 by high-speed stirring. The mixture was then milled and dispersed by a three roll mill 3 times to form a conductive paste. The conductive paste was screen printed on a polysilicon substrate, and then sintered in an oven at 800° C. for 30 minutes to form an electrode on the substrate. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 4.

Comparative Example 13

(39) 40 g of copper powder (mean particle size of 0.1 μm, prepared according to H. T. Zhu et al., Journal of Crystal Growth, 270, 722 (2004)), 60 g of silver powder (mean particle size of 2.0 μm, commercially available from Ferro), 5 g of glass powder (mean particle size of 3.0 μm, commercially available from Okamoto Glass Co., Ltd.), and 1.0 g of carbon nanotube (specific surface area of about 200 m.sup.2/g, MWCNT commercially available from Seedchem Company Pty., Ltd.) were mixed with 15 g of the binder A in Example 1 by high-speed stirring. The mixture was then milled and dispersed by a three roll mill 3 times to form a conductive paste. The conductive paste was screen printed on a polysilicon substrate, and then sintered in an oven at 800° C. for 30 minutes to form an electrode on the substrate. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 4.

(40) TABLE-US-00004 TABLE 4 Silver Copper powder, powder, Additive mean mean ratio (on the particle particle size basis of the Glass Volume size of of 0.1 μm copper powder Binder resistivity 2.0 μm (g) (g) Additive powder) (g) (g) (μΩ-cm) Comparative 60 40 — — 5 15 >10{circumflex over ( )}4 Example 11 Comparative 60 40 Carbon 1.25% 5 15 5200 Example 12 nanotube (0.5 g) Comparative 60 40 Carbon 2.50% 5 15 4850 Example 13 nanotube (1.0 g)

(41) As shown in Table 4, a copper powder having an overly small particle size would largely increase the volume resistivity of the electrode made from the conductive paste.

Comparative Example 14

(42) 80 g of copper powder (mean particle size of 10 μm, commercially available from DOWA), 20 g of silver powder (mean particle size of 2.0 μm, commercially available from Ferro), and 5 g of glass powder (mean particle size of 3.0 μm, commercially available from Okamoto Glass Co., Ltd.) were mixed with 15 g of the binder A in Example 1 by high-speed stiffing. The mixture was then milled and dispersed to form a conductive paste. The conductive paste was screen printed on a polysilicon substrate, and then sintered in an oven at 800° C. for 30 minutes to form an electrode on the substrate. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 5.

Comparative Example 15

(43) 80 g of copper powder (mean particle size of 10 μm, commercially available from DOWA), 20 g of silver powder (mean particle size of 2.0 μm, commercially available from Ferro), 5 g of glass powder (mean particle size of 3.0 μm, commercially available from Okamoto Glass Co., Ltd.), and 1 g of graphene (specific surface area of about 500 m.sup.2/g, prepared according to H. A. Becerill, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and Y. Chen, ACS Nano 2, 463 (2008)) were mixed with 15 g of the binder A in Example 1 by high-speed stirring. The mixture was then milled and dispersed to form a conductive paste. The conductive paste was screen printed on a polysilicon substrate, and then sintered in an oven at 800° C. for 30 minutes to form an electrode on the substrate. The adhesive force between the electrode and the substrate was 5 B. The volume resistivity of the electrode is tabulated in Table 5.

(44) TABLE-US-00005 TABLE 5 Additive ratio (on the Silver Copper basis of the Glass Volume powder powder copper powder Binder resistivity (g) (g) Additive powder) (g) (g) (μΩ-cm) Comparative 20 80 0 0 5 15 >10{circumflex over ( )}5 Example 14 Comparative 20 80 Graphene 1.25% 5 15 7850 Example 15 (1 g)

(45) As shown in Table 5, an overly high ratio of the copper powder would increase the volume resistivity of the electrode made from the conductive paste.

(46) The conductive paste compositions in the above embodiments can be applied in a sintering process at a high temperature for a silicon-based solar cell. The metal particles (copper/silver) with different particle sizes can be mixed to enhance the filling density of the conductive pastes and conductivity of the electrodes made from the conductive pastes. The carbon material with a high specific surface area (e.g. graphene or carbon nanotube) can be added to the conductive paste to enhance its anti-oxidation ability. The formula may reduce the amount of silver in the conductive pastes, thereby reducing cost. The disclosure may solve the conventional problem wherein the copper in the conductive paste is easily oxidized during high-temperature processes. As such, the conductive paste compositions of the disclosure are excellent in the high-temperature sintering process for preparing solar cells.

(47) It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed methods and materials. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.