Homework screen printing

School of PV and RE Engineering

Lecture 8 – Screen Printing and Rear Al Metallisation

Andre Augusto

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Lecture Overview

1. Metallisation Overview 2. Screen-Printing And Drying 3. Rear Surface Al Electrode 4. Front Surface Ag Grid Electrode 5. Revolutions in Screen Printed Solar Cell Industry 6. Localised Rear Contacts 7. Future of SP Metallisation

Today

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Screen-Printed Si Solar Cells Saw-damage removal

Texturing

Emitter diffusion

PSG Removal

Edge isolation

SiNx antireflection coating

Al rear SP

Ag front contact SP

Co-firing

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• Metal electrodes enable current to be collected from cells

• Aluminium (Al) metal paste is used for the rear surface to make electrical connection to the rear p-type wafer. The Al usually covers most of the rear surface.

• Silver (Ag) metal paste is used for the front surface to make connection to the front n-type diffused emitter. The Ag is usually patterned with an “H” shaped grid pattern.

• Screen printing is used to place the metal onto the surfaces. Thus, the metal is initially formed into a viscous liquid “paste” that can be screen printed.

• A “firing” process then sinters and alloys the metal paste together and to the silicon wafer so that a low- resistance, “ohmic” contact is formed with good adhesion to the silicon wafer

• The “firing” process also forms an Al-Si alloy at the rear surface that is beneficial in reducing recombination at the rear surface (so-called “Back Surface Field”)

Metallisation Overview

Screen printing screen “H” grid pattern of front electrode

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• Current practice is to screen print an aluminium (Al) paste on the rear surface and then fire this paste so that it forms an Al-Si alloy at the rear surface

• The Al-Si alloy functions to (1) dissolve enough silicon to wipe out any n- diffused layers on the rear surface, (2) make electrical contact to the rear of the p-type wafer and (3) form a p++ heavily doped layer at the rear that serves as a minority carrier Back Surface Field (BSF).

n+ ~ 0.3-0.5 um

p ~ 170 um

p++ ~ 8 um

Al Back Surface Field (BSF)

n-diffused layer

Si wafer

p++ Al-Si alloy layer

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• The BSF has an associated built-in electric field (that works similarly to the n-p junction on the front surface, albeit to a smaller degree): Ø Repels electron that impinge on the rear surface; and therefore Ø Reduces recombination at the rear surface.

• Lower recombination à Higher VOC • Higher rear collection probability (of long wavelength light) à Higher JSC

n+ ~ 0.3-0.5 um