KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [1]

Lead-free Silver-paste for Fine-line Printing with Ultra-fine Mesh Screen

Masashi Nakayama1, Nobuo Ochiai1, Kenichi Harigae1,Koji Honda2, Shinya Kitada2, Hideaki Hayashi2, Kyotaro Nakamura3, Mari Aoki3, Isao Sumita3, Yoshio Ohshita3

1Kyoto Elex Co., Ltd. 1,Oogawaracho,Kisshoin,Minami-ku,Kyoto 601-8391, Japan2Asada Mesh Co., Ltd. 23-7, Shindo-4, Matsubara, Osaka 580-0015, Japan3Toyota Technological Institute 2-12-1 Hisakata, Tempaku-ku, Nagoya 468-85111, Japan

セル

写真

Outline

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [2]

■Motivation & Challenges

■Results of under 50μm line

■Comparison (Conventional vs Our Challenges)

■Ultra-fine Mesh Screen

■Lead-Free Ag Paste

■ Results of under 40μm line

■ Results of under 30μm line

■ Results of high sheet resistance emitter cｅｌｌs

■Conclusion

Motivation and Challenges

120Ω/□

http://www.itrpv.net/

30 μm

Finger width

Emitter Sheet Resistance

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [3]

3rd International Technology Roadmap for Photovoltaics (ITRPV) Results 2011

Motivation and Challenges

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [4]

①Fine Line (～30μm)

②Shallow Emitter (～120Ω/□)

③Lead-Free

New Lead-Free Ag Paste

Ultra-Hard HS-D Mesh

Step1: Under 50 μmStep2: Under 40 μmStep3: Under 30 μm

Metallization using screen printingon solar cell already reached the limit?

Leading spec in mass-production

Printed finger width

Around 60μmwith Double Printing

Our Challenges

Asada Mesh

Kyoto Elex

Pb

with Double Printing

Main requirementsfor Front side Ag Paste

Key words

Snap-Off Distance Control for Registration

Cross-

SectionWidth Aspect

Ave. Ave. Ave. Max. Min.

µm2 µm µm µm µm

mc-Si 403 47.2 15.1 18.1 12.1 0.32

CZ-Si 383 47.2 14.5 18.2 11.1 0.31

20μm

pattern

40+40µm

90finger

3busbar

HS-D500φ19

em20µmCL

Height ave.

/Width

Line Height

Cell No.Mesh Emulsion

Fired Finger Line (3D Laser Confocal Microscope OLS4000)

40μm aperture ⇒ Printed line width is under 50μm (45～50μm)

40+40μmDoubleon mc-Si

40+40μmDoubleon CZ-Si

Ag Paste： KYOTO ELEX

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [5]

STEP1: Under 50μm Line (40+40μm aperture printing) with 500 mesh

EL image mc-M552-4 mc-M552-5 mc-M552-6

CZ-M552-4 CZ-M552-5 CZ-M552-6

FF=0.802、η=17.58% FF=0.803、η=17.51% FF=0.797、η=17.54%

FF=0.796、η=18.73% FF=0.803、η=18.88% FF=0.803、η=18.85%

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [6]

STEP1: Under 50μm Line (40+40μm aperture printing) with 500 mesh

IV measurement (※ KYOTO ELEX IV Condition)

【Condition】

・Squeegee Speed：200mm/s・Ag Paste：Kyoto Elex (Lead-Free)※1st Print⇒350Pas、2nd Print⇒250Pas・6inch mc-Si Wafer (70Ω/□)・6inch CZ-Si Wafer (80Ω/□)

・Mesh： HS-D500φ19 CL (Asada Mesh)・Emulsion： 20μm ・Pattern： 40+40μm90finger3busbar・IR furnace：PV309(BTU) ⇒ firing Peak Temp:780℃

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [7]

Wafer

Isc

[A]

Jsc

[mA/cm2]

Voc

[V]

FF

η

[%]

Ag Wet

[g/cell]

8.460 34.77 0.631 0.802 17.58 0.093

8.454 34.74 0.628 0.803 17.51 0.090

8.524 35.03 0.628 0.797 17.54 0.095

8.858 37.22 0.633 0.796 18.73 0.107

8.846 37.17 0.632 0.803 18.88 0.095

8.837 37.13 0.633 0.803 18.85 0.094

Ave.

Wafer

Isc

[A]

Jsc

[mA/cm2]

Voc

[V]

FF

η

[%]

Ag Wet

[g/cell]

mc-Si 8.479 34.84 0.629 0.801 17.54 0.093

CZ-Si 8.847 37.17 0.632 0.801 18.82 0.099

mc-Si

CZ-Si

STEP1: Under 50μm Line (40+40μm aperture printing) with 500 mesh

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [8]

Comparison of Conventional Screen Printing with Our ChallengesConventional screen printing on solar cell

Our Challenges

Screen

Paste

Problem

Screen

Paste

Aims

①Screen life of high open area rate (60%) is short. ②Printing resolutions of high open area rate (60%) is bad.③High viscosity paste can’t print with high-mesh(=low open area rate)④So, we can’t print under 50 μm line in mss-production condition.

Higher open area rate (60%) delivers better paste release.

High viscosity paste has high silver content and gives good aspect rate.

①Achievement of minimum cross-section for high FF by double printing②Improvement of printing resolutions and optimization of under 50 μm printing

①3 times strength of standard stainless wires②40% open area rate③Ultra-fine mesh (#500/#650/#900) for target finger line width

Low viscosity and various control for screen of 40% open area rate and ultra-fine mesh to print under 50μm finger line

SEM Image of Mesh ①

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [9]

60%

HS-D500φ19CLHS-360φ16CL MS-400φ19CL

50% 40%Open area rate 40% ⇒①Control paste flow ②Printing Resolutions ③Screen Durability

oursolution

Select Lower Open Area Ratio Mesh for Fine Line Printing

Low open area rate = High mesh count

Twill

100μm 100μm 100μm

SEM Image of Mesh ①

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [10]

#500

HS-D500φ19CL

#650 #900Target mask aperture of finger width ≒ Wire Diameter φ×2

HS-D650φ14CL

Select Thinner Wire and Higher Mesh Count for Fine Line Printing

trial stage

MS-D900φ12CL

40μmaperture

30μmaperture

20μmaperture

100μm 100μm 100μm

WaferRsheet

[Ω/□ ]Busbar Finger

number

Printed FingerWidth[μm]

Printing Mesh

60

70

80

90

100

120

conventional

2Bus

3Bus

4Bus

50

5Bus

0Bus

Back contact

MWT/EWT/IBC

72

80

90

120

150

conventional

80

70

60

conventional

Double

Triple?

Single290φ20CL

HS-D500φ19CL

325φ23

325φ16CL

360φ16CL

HS-D650φ14CL

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [11]

210

conventional

MS-D900φ12CL

50

40

30

Total Optimization of Design and Process for Shallow Emitter

Selection of mesh for target finger width

MaskAperture

[μm]20 25 30 35 40 45 50

PrintedWidth[μm]

30 35 40 45 50 55 60

Mesh MS-D900φ12 HS-D650φ14 HS-D500φ19

Emulsion [μm] 10～15 15～20 20～25

Screen Spec

Other Screen Design

・Finger number・Busbar・Taper Spec

mass-production next generation

Ag Paste

・Fire Through・Viscosity・Thixotropy

Wafer Spec

・Sheet resistance(～120Ω/□)・LDE (Low Doped Emitter)・Texture Size・CZ-Si / mc-Si

Total Optimization

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [12]

Target mask aperture of finger width ≒ Wire Diameter φ×2

Fire through control with lead-free glass frit and special other component

Ag Paste Component

Ag Powder

Glass Frit

Organic Vehicle

Solvent

Additive

Front side Ag

BSF（P+ layer）

Si（N layer）

ARC(SiNx)

Rear side AlStructure of c-Si Solar Cell

Si（P layer）

Fire Through

※BSF layer（Back Surface Field）

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [13]

Lead-Free Ag Paste of KYOTO ELEX

～120Ω/□wafer available

mild and selective attack without shunt

■Mission for paste ②

■Mission for paste ①

Severe rheology control of the paste for ultra high mesh under 50μm printing

Pb

Fired Finger Line Ag Paste： KYOTO ELEX

30+30μmDoubleon mc-Si

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [14]

Dried Line

Cross-

SectionWidth Aspect

Ave. Ave. Ave. Max. Min.

µm2 µm µm µm µm

30+30µm

150finger3busbar

HS-D650φ14

em15µmCLmc-Si 305 36.5 12.2 16.4 8.6 0.33

pattern MeshHeight ave.

/Width

Wafer

Line Height

Paste viscosity in 2nd printing of Double Print

Fired Line

250Pas 200Pas

STEP2: Under 40μm Line (30+30μm aperture printing) with 650 mesh

6inch mc-Si Wafer （Z10）

34.2

34.3

34.4

34.5

34.6

34.7

34.8

34.9

Viscosity of the paste for the 2nd printing

Jsc[m

A/cm

2]

250Pas(Ref) 200Pas 150Pas

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [15]

6inch mc-Si Wafer （Z10）

0.775

0.780

0.785

0.790

0.795

0.800

0.805

Viscosity of the paste for the 2nd printing

FF

250Pas(Ref) 200Pas 150Pas

Lower viscosity of paste for 30μm aperture in 2nd printing 250 ⇒ 200 Pas①line forming improvement②FF improvement

STEP2: Under 40μm Line (30+30μm aperture printing) with 650 mesh

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [16]

New lead-free Ag paste is available for 30μm both SP and DP with 900 mesh screen.

STEP3: Under 30μm Line (20+20μm aperture printing) with 900 mesh

【Printed Condition】

Mesh：MS-D900φ12em10μmCLAg Paste：KYOTO ELEX

Squeegee Speed：200mm/sSubstrate：CZ-Si textured wafer

20μm apertureSingle Print

20+20μm apertureDouble Print

(Lead-Free)

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [17]

Test Result High FF for high sheet resistance emitter

【Printed Condition】

Ag Paste:KYOTO ELEX(Lead-Free Ag Paste)Screen：

HS-D650Φ14em15μmCL(Ultra high mesh)Pattern:30+30μm150finger3busbar(Double Printing)

Emitter sheet resistance60⇒120Ω/□

・Keep high FF(≒0.800)・Increase Jsc

Pb

Conclusion

KYOTO ELEX CO., LTD 4th Metallization Workshop in Constance on 7-8 May,2013 [18]

・We’ve been able to achieve under 50μm line screen printing on solar cell wafer with properly tuned combination of ultra-High meshand new lead-free Ag paste.

・Every target line width from mass-production to next-generation, there are 3 steps which we propose the idea of screen specand they gives good fine line printing.

・In addition to ultra fine line, the new lead-free Ag paste suggests potential to be capable of adapting to over 100Ω/□ high sheet resistance cells (～120Ω/□).

・As of today, we get a part of the solutions toward 30μm line screen printing and 120Ω/□ cell available to get high performance solar cells. The end goal requires “ total optimizationof solar cell process”, not only individual improvement.