Ultra Fine Line Printing for

Silicon Solar Cells…..

Mesh Screens or Metal Stencils?

Tom Falcon 25/10/11

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Agenda.

1. Background.

2. Screen & Stencil Types.

3. Processes.

4. Results.

5. Conclusions.

3

Background -

Solar Cell Structure

Sun Side

Rear Side

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Low A.R. Results in:

• High Shadowing Loss

• Higher Silver Costs

High A.R. Results in:

• Low Shadowing Loss

• Lower Silver Costs

Background – Aspect Ratio

• Conductors typically shade 6 to 8% of silicon area.

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Background - Silver Price.

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Background -

Predicted Finger Width.

Source: ITRPV Roadmap 2011

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Drivers for this work….

• Reduce Shadowing Losses.

• Reduce Silver Consumption.

• Enable Higher Sheet Resistance.

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Agenda.

1. Background.

2. Screen & Stencil Types.

3. Processes.

4. Results.

5. Conclusions.

9

Screen & Stencil Tests.

• Ultra-Fine Wire Mesh Screens.

• Double Layer “Hybrid” Screen.

• Single Layer Stencils for “Dual Print”.

• Ultra-Fine Wire Screens For Print on Print.

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Mesh Screen Guidelines

Generally Speaking…..

• Percent Open Area Influences Aspect Ratio. Higher % Open Areas Have Greater Paste Transfer Efficiency

• Wire Diameter Influences Min Opening Width. Width ≥ Wire Ø x 2.5

(Wire Diameter Also Influences Screen Strength / Lifetime)

• Mesh Count Influences Uniformity & Strength. Higher Wire Counts Improve Height Uniformity and Add Tensile Strength.

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Ultra Fine Line Meshes

Mesh Count

Wire

Diameter

% Open

Area

Theoretical

Minimum

Opening

300 20µm 60% 50µm

400 18µm 51% 45µm

350 16µm 60% 40µm

380 14µm 62% 35µm

590 13µm 49% 32.5µm

840* 11µm 40% 27.5µm * Yet to be tested.

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Ultra-Fine Wire

Mesh Screens.

30µm Wide Aperture. 380 Mesh with 14µm Wire Diameter.

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Ultra-Fine Wire

Mesh Screens.

25µm Wide Aperture. 380 Mesh with 14µm Wire Diameter.

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Hybrid Solar Screen

Top Layer is High Tensile

Etched Stainless (Grey).

Bottom Layer is

Photo-Imageable Polymer.

Photo

Sketch

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30µm Thick Nickel Stencil.

30µm Aperture in 30µm Thick Nickel Stencil.

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25µm Thick Nickel Stencil.

25µm Aperture in 25µm Thick Nickel Stencil.

62% 100%

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Agenda.

1. Background.

2. Screen & Stencil Types.

3. Processes.

4. Results.

5. Conclusions.

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Single Print Screens.

Fingers & Busbars all in one print process.

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Dual Print:

Screen then Stencil.

1st Print + 2nd Print = Whole Print

Mesh Screen Eformed Stencil

Busbars Only Fingers Only Full Grid

Standard mesh screen and single layer e-formed nickel stencil.

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Print on Print

Two Mesh Screen Designs…

1st = Fingers Only + 2nd = Fingers & Busbars.

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Agenda.

1. Background.

2. Screen & Stencil Types.

3. Processes.

4. Results.

5. Conclusions.

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Single Print, Mesh Screen

380 Mesh, 14µm Wire.

30µm Opening.

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Single Print, Mesh Screen

380 Mesh, 14µm Wire.

25µm Opening.

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Hybrid Screen

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30µm Metal Stencil

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25µm Metal Stencil

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25um Stencil

32.7µm

15

10

5

Profile Section from 25um Stencil Print

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Print on Print

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Print on Print

Profile Section from Print on Print

49.7µm

25

20

15

10

5

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Results Table:

Line Width

Line Height

Height Range

Aspect Ratio

Single Print

Mesh Screen

380/14

45µm

11.4µm

4.9µm

0.254 Single Print

Hybrid Screen

40um Thick

61µm

17.4µm

5.3µm

0.285 Dual Print

Metal Stencil

30µm Thick

45µm

17.6µm

2.0µm

0.391 Dual Print

Metal Stencil

25µm Thick

35µm

14.0µm

2.8µm

0.400 Print on Print

Mesh Screen

380/14

50µm

26.9µm

8.7µm

0.538

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Agenda.

1. Background.

2. Screen & Stencil Types.

3. Processes.

4. Results.

5. Conclusions.

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Conclusions.

• <50µm Lines Possible with Both Screens & Stencils.

• 380 Mesh with 14µm Wire is Preferred for Mesh Screen.

• Dual Print Gives Good Height & Superb Uniformity.

• Print on Print Gives the Best Possible Aspect Ratio.

• Dual Print Gives Best Ultra-Fine Lines (~35µm).

• Finer Wire Diameters Needed to Fulfil Screen’s Potential.

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Questions?….

For further information please contact:

Tom Falcon.

Tel: +44 1305 208415

Email: tfalcon@dek.com

http://www.deksolar.com